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.

d) the number of neutrons plus the number of protons