Longitudinal waves compress the medium they travel though. One example of a longitudinal wave is a

It is sound waves

(I did USAtestprep)

Answer: Waves in compressed string

Explanation:

Longitudinal waves are waves in which the vibration of the medium is going parallel to the direction of the waves produced by the medium.

Longitudinal wave can occur in compressed string. A compressed string displaced horizontally will transmit wave in the same horizontal direction. That's why they are longitudinal waves.