Why is a sound wave called a longitudinal wave?

Hint: Sound waves in all (and any fluid medium) use longitudinal waves because particles of the medium through which the sound is transported vibrate parallel to the direction that the sound wave moves.

Complete step by step answer:
Basically Sound wave is a mechanical disturbance. Disturbance is the to and fro motion of the particles. In which sound energy propagates from one place to another place. Sound wave is the wave which requires a medium so it is known as mechanical wave.
Waves in which particles of the medium move in a direction parallel to the direction of the wave or in the direction of energy transport are called longitudinal waves.
In a sound wave, the particles of the medium vibrate in the direction of propagation disturbances or the particle of medium.
In sound waves there are two phenomena by which it is produced i.e. compressions (high pressure) and rare fractions (low pressure) in the air.
The air particles vibrate parallel to the direction of propagation.
Hence a sound wave is called a longitudinal wave.
Whereas the wave in which the particles of the medium vibrate about their mean position in the direction perpendicular to the direction of propagation of the wave is called the transverse wave. Example waves formed on the surface of water on dropping a piece of stone in it.

Note:
Basically longitudinal waves are waves in which the displacement of the medium is in the same direction as, or the opposite direction to the direction of propagation of the wave.