Ionic solids or compounds are formed by the transfer of electrons from one atom to another atom. Hence there is always the formation of anion and cation due to opposite charges electrostatic force of attraction formed between them which holds them together.

Diamond has a very high melting point (almost 4000°C) due to very strong carbon-carbon covalent bonds have to be broken throughout the structure before melting occurs. Diamond is very hard.

• Amorphous solids are isotropic in nature.The structure of amorphous solids is similar to that of liquids.
• Hence these are also called pseudo solids or super cooled liquids.
• Isotropy of amorphous solids is due to same irregular arrangement of constituent particles along all the directions.

• Ice is a hydrogen bonded molecular solid.
• It is a crystalline substance in which the constituent particles are molecules.
• These molecules are held together by hydrogen bonds.

Crystalline solids are anisotropic in nature, that is, some of their physical properties like electrical resistance or refractive index show different values when measured along different directions in the same crystals. This arises from different arrangements of particles in different directions.

• Due to random organization of particles, amorphous solids have same physical properties along all directions, or are isotropic.
• Random organization of particles also results in unequal bond strengths.
• Upon cooling, amorphous solids turn into brittle glass-like state from a flexible rubber-like state. This is called glass-transition.

A diamond is not considered as a molecule because each carbon atom is covalently bonded with four other carbon atoms. This is what makes diamond a network solid.

Molecular solids consist of atoms or molecules held to each other by dipole–dipole interactions, London dispersion forces, or hydrogen bonds, or any combination of these. 

• An amorphous or non-crystalline solid is a solid that lacks the long-range order that is characteristic of a crystal.
• In brief, an amorphous solid is any noncrystalline solid in which the atoms and molecules are not organized in a definite lattice pattern. Such solids include glass, plastic, and gel.
• Quartz glass (SiO₂) is an amorphous solid due to its short range order of constituent particles. Note Quartz is a crystalline solid while quartz glass is an amorphous solid.

Amorphous solids become crystalline on ageing e.g. ancient glass appears milky due to crystallization.

Amorphous solids are isotropic that is they have identical properties in all directions, whereas crystalline solids are anisotropic that is they have different properties in different directions.

Potassium chloride is an ionic compound.
Since, potassium is a metal and chlorine is a non-metal and if a metal is mixed with a non-metal it gives an ionic compound. 

Non polar Molecular Solids:

• They are composed of either atom.
• H₂, Cl₂ and I₂.are some of the typical example.
• In non-polar Molecular solids the comprised atoms or molecules are held together by weak dispersion forces or London forces.
• They are soft and non-conductors of electricity.
• They have low melting points and usually exist in liquid or gaseous state at room temperature and pressure.

Silicon due to its catenation property form network solid. Catenation is the ability of an atom to form bonds with other atoms of the same element. The compounds of silicon are reactive and not stable.

Magnitude of the repulsion rises very rapidly as the distance separating the molecules decreases. This is the reason that liquids and solids are hard to compress.

In a metal, the valence electrons are loosely held. They leave their “own” metal atoms, forming a "sea" of electrons surrounding the metal cations in the solid.

Thus, metals are good conductors of electricity.

Dry ice is solid carbon dioxide which is generally used in packing frozen items is an example of molecular solid.

In diamond, carbon atoms are joined together by strong covalent bonds tetrahedrally.

Chemists have learned over time that even within a group of solids that share a similar structure (i.e., lattice), other differences affect the way the solid behaves. The bonds and molecular composition of a solid influence its conductivity, malleability, solubility, density and other factors.

• In order to conduct electricity a substance must have charge particles, such as electrons and ions, that are free to move freely through it.
• In the solid state, ionic compounds such as sodium chloride and potassium chloride have their ions fixed in position and therefore these ions cannot move so solid ionic compounds cannot conduct electricity.
• However, in the molten state, ions in ionic compounds are free to flow and therefore molten sodium chloride can conduct electricity.

• Low boiling point is the property which is not exhibited by metallic solids. The reason is metallic solid's atoms are bonded with strong molecular force of attraction coz of which the bond between the atoms doesn't breaks easily.
• Very high temperature is required to break the bond between the atoms. though the boiling point varies from element to element. so, as a result, metallic solids have a strong molecular force of attraction, they do not have a low boiling point. 

Ionic bond is much stronger than covalent bond because it involves complete transfer of electrons because of which there is formation of cation and anion and there exist huge electrostatic forces of attraction. They also have high melting and boiling point which proves that the ionic bond is very strong.

Crystalline compounds are those which particles/molecules are arranged in proper order as in ice particles/water molecules are arranged in cage like ordered manner so this ordered structure is called as crystalline compound.

• Quartz (SiO₂), or silicon dioxide also known as silica has a giant covalent structure. In SiO₂, each silicon atom is covalently bonded to four oxygen atoms.
• Each oxygen atom is covalently bonded to two silicon atoms and has an extended covalent structure similar to diamond.
• The overall ratio is two oxygen atoms to each silicon atom, giving the formula SiO₂, 
• Hence, option C is the correct answer.

The point at which the atoms may be present on the unit cell is termed as lattice point. It shows the position of atoms in crystal.