Unlike a transverse wave, the disturbance in a longitudinal wave is

a) in the opposite direction of the wave motion.

b) in a circular motion.

c) perpendicular to the wave motion

d) parallel to the wave motion.

In a longitudinal wave, disturbance is parallel to wave motion, which is not similar to transverse wave.

Option D

Explanation:  

In a longitudinal wave, particles in the "medium move parallel to direction of propagation of wave".  

Example of Longitudinal wave: Sound wave

In transverse wave motion of particle is perpendicular to direction of wave but in the longitudinal wave the "medium move in same direction of wave".  

Example of Transverse wave: Ripples on surface of water.

labor day, patriot day, leaves start to change colors then fall from the tree

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.