Awave which needs a medium (solid, liquid, gas) in order to propagate itself.

The correct answer to the question is: Sound wave.

EXPLANATION:

Before going to answer this question, first we have to understand longitudinal wave.

A longitudinal wave is a mechanical wave in which the direction of propagation of wave is parallel to the direction of particle vibration.

This wave propagates through a medium in the form of compression and rarefaction.

Compression are the regions of high pressure zones where the particles of the medium are closely aggregated to each other.

Rarefaction is the region of low pressure where particles of the medium are not so close to each other as compared to compression.

The longitudinal wave can not propagate in space. It always needs a medium for it's propagation. The medium may be solid, liquid or gas .

For instance, sound wave is a longitudinal wave.

Hence, the correct answer of this question is sound wave which needs a medium for it's propagation.