Cardiac muscle is highly organized and bears striking similarities to skeletal muscle in terms of its basic morphology.

Functionally cardiac muscles resemble unstriped muscles, as both of them are involuntary muscles and have the function of contraction and relaxation.

Fascia (or "deep fascia") is a layer of thickened connective tissue that covers the entire muscle and is located over the layer of epimysium.

Myofibrils are composed of long proteins including actin, myosin, and titin, and other proteins that hold them together. These proteins are organized into thick and thin filaments called myofilaments, which repeat along the length of the myofibril in sections called sarcomeres. Muscles contract by sliding the thick (myosin) and thin (actin) filaments along each other.

The globular head is an acitve ATPase enzyme and has binding sites for ATP and active actin sites.

In the context of muscular contraction, a cross-bridge refers to the attachment of myosin with actin within the muscle cell.
All muscle types contract by cross-bridge cycling i.e. repeated attachment of actin and myosin within the cell.

Hence, a cross-bridge is formed during muscle contraction. 

A refractory period is a period of time during which an organ or cell is incapable of repeating a particular action, or (more precisely) the amount of time it takes for an excitable membrane to be ready for a second stimulus once it returns to its resting state following an excitation.

Sliding Filament Theory of Contraction:
During muscle contraction, the thin actin filaments slide over the thick myosin filament.
When Calcium is present the blocked active site of the actin clears.

Purkinje fibers: The specialized cardiac muscle fibers forming a network in the ventricular walls that conduct electric impulses are responsible for the contractions of the ventricles.

Cardiac muscle cells have a high mitochondrial density in comparison to other muscles fiber, which allows them to produce adenosine triphosphate (ATP) quickly, making them highly resistant to fatigue.

The SA node, also known as the sinus node, which is a group of specialized muscle fibres is the heart's natural pacemaker. It consists of a cluster of cells that are situated in the upper part of the wall of the right atrium.

Rigor mortis is stiffening of the joints and muscles of a body a few hours after death, usually lasting from one to four days.
Rigor mortis usually sets in within four hours, first in the face and generally smaller muscles.

Red muscles are skeletal muscles that are rich in capillaries, myoglobin, and mitochondria. Capillaries are very small blood vessels that deliver oxygenated blood to the muscles. This oxygen is taken up by a protein called myoglobin. The myoglobin then helps provide the oxygen to the mitochondria, the sort of power plant of the cell. They use oxygen, among other things, to produce a lot of energy.

• Unstriped muscles have no striation, so they are called smooth muscles. 

• They are present in the internal body organs, so they are called visceral muscles.

• They are not voluntary control like skeletal muscles, so they are called Involuntary muscles.

Sarcostyles are myofibres and are one of many contractile filaments that make up a striated muscle fiber.

A sarcomere is the basic contractile unit of muscle fiber.

Each sarcomere is composed of two main protein filaments, actin and myosin, which are the active structures responsible for muscular contraction.

Actin protein is an important contributor to the contractile property of muscle and other cells.

Epimysium of Muscles are made up of white fibrous connective tissue while Perimysium and endomysium are made up of areolar tissue.

• Myosin filaments are thick, consisting mainly of the protein myosin. The thin actin filaments consist mostly of the protein actin; they also contain the proteins tropomyosin and the troponin complex that regulate the actin filaments interaction with myosin filaments.
• The filaments overlap lengthwise in the muscle fibres, producing the pattern of transverse bands or striations characteristic of striated muscle.
• The striations of striped muscles result from the fact that the thick and thin filaments have different refractive indices in the light microscope. Thus, myosin filaments appear dark.

An isotonic contraction occurs when tension in the muscle remains constant despite a change in muscle length. Lifting an object off a desk, walking, and running involve isotonic contractions.

The Gluteus Maximus is the largest muscle in the human body. It is large and powerful because it has the job of keeping the trunk of the body in an erect posture. It is the chief antigravity muscle that aids in walking up stairs.

In the Cori cycle, lactate accumulated in the muscle cells is taken up by the liver. The liver performs a chemical process known as gluconeogenesis, to convert lactate back to glucose. This first converts lactate to pyruvate, and then finally into glucose.

The papillary muscles are muscles located in the ventricles of the heart. The papillary muscles constitute about 10% of the total heart mass.

Birds power flight primarily by large pectoralis muscles that depress the wings at the shoulder.

The external muscles of the eye are conspicuously large and strong in relation to the small size and weight of the eyeball. It is frequently said that they are the strongest and fastest contracting muscles due to the constant blinking action they have to do.

 Eye movements do require high-speed movements, and eye muscles are exercised continuously even during rapid eye movement of sleep.

In all myosins, the head domain is a specialized ATP-ase that is able to couple the hydrolysis of ATP with motion. A critical feature of the myosin ATP-ase activity is that it is actin-activated. In the absence of actin, solutions of myosin slowly convert ATP into ADP and phosphate.

There are total 639 muscles in human body.

Smallest muscle is ‘stapedis' found in middle ear attached with stapes (smallest bone).

Largest muscle is ‘glutious maximus' also it is the most powerful muscle of human body.

During pregnancy, the smooth muscle of uterus expands and attains maximum size and thus becomes largest smooth muscle.

The strongest muscle in human beings is the masseter muscle. The muscle masseter is a thick muscle in the cheek and is situated at the back of the jaw. During chewing it is responsible for opening and closing the jaw.

The neural tube ectoderm gives rise to the retina, the iris and ciliary body epithelia, the optic nerve, the smooth muscles of the iris, and some of the vitreous humor.

Multi-unit smooth cells can be found in the trachea, the large elastic arteries, the iris of the eye, in the lungs, erectile tissues of hair follicles ( Arrector pili of the dermis), and the vas deferens in the male genital tract.

Multi-unit smooth muscle activity can be initiated by hormones, but it is not much affected by rapidly stretching the muscle.

Cardiac muscle is found only in the walls of the heart. When cardiac muscle contracts, the heartbeats and pumps blood.

• Cardiac muscle contains a great many mitochondria, which produce ATP for energy. This helps the heart resist fatigue.
• Contractions of cardiac muscle are involuntary, like those of smooth muscle.
• Cardiac muscle, like skeletal muscle, is arranged in bundles, so it appears striated, or striped.

Hence, option D is correct.