Which of the following describes a longitudinal wave?

A longitudinal wave is characterized by the movement of the medium's particles in a direction that is parallel to the direction of the wave's travel. This means that as the wave propagates through a medium, such as air or water, the particles of that medium oscillate back and forth along the same line that the wave is moving. The compressions and rarefactions created by this movement define the structure of a longitudinal wave.

In contrast, waves that vibrate perpendicular to their direction of travel, such as transverse waves, create a different behavior where the particle motion is sideways relative to the wave's direction. Circular motion is not characteristic of either longitudinal or transverse waves but rather describes the movement of some physical systems or phenomena, such as waves on the surface of water under specific conditions. Thus, the description of a wave vibrating parallel to its direction of travel correctly identifies the nature of longitudinal waves.