• Sound waves need to travel through a medium such as a solid, liquid, or gas. The sound waves move through each of these mediums by vibrating the molecules in the matter.

• Sound travels fastest in solids because the constituent particles are most closely/tightly packed. Sound is a longitudinal wave, which means that sound waves travel through the vibrations of individual particles of matter (compressions and rarefactions) which are along the direction on propagation. Only the disturbance is transmitted forward, without requiring the particles to actually move themselves. The neighboring particles in solids are very close to one another, resulting in faster, high frequency, back and forth vibrations of particles. Thus the disturbance/sound can travel more quickly from one point to another, as opposed to liquids and gases where particles are spaced farther apart.

• Hence, Sound travels fastest through solids, slower through liquids and slowest through gases.

• Sound will travel faster in steel than the wood because the particles are more closely packed in steel in comparison of wood.

Option C is correct.

Light year, Astronomical Unit and Par sec are the units used to measure very large distances whereas slug is a unit to measure heavy masses. 1 slug = 14.57 kg

Option D is correct.

Assume 'l' and 'A' be the length and area of cross-section of the conductor respectively.

Current density,

 =2.5 x 10⁶ Am^-2

Electric field,  = 15 V m^-1

Now, resistivity

Option A is correct.

The specific heat is the amount of heat per unit mass required to raise the temperature of a body by one degree Celsius.

(Where Q is the heat transfer, c is the specific heat, m is the mass of the body,  is the change in temperature of the body)

Hence, by the definition of the specific heat, we can clearly say that it is a material property and depends only upon the nature of the substance.

Hence, option D is correct.

As it is given that a man pushes a wall, then he will exert a force on the wall but fails to displace it.

And we define the work- done as the dot product of the force applied and displacement.

i.e, W = F.ds

But here, displacement (ds) = 0

Hence, the work-done by man = zero

So, option C is correct.