Which of the following describes a longitudinal wave?

A longitudinal wave is characterized by the movement of particles in the medium in the same direction as the wave is traveling. This means that as the wave propagates, the particles of the medium oscillate back and forth along the same axis as the wave motion. An example of a longitudinal wave is a sound wave, where compressions and rarefactions move through the air in the same direction as the sound travels.

The other options do not accurately describe the nature of a longitudinal wave. For instance, a wave that moves in a direction perpendicular to the wave motion describes a transverse wave, where the oscillations occur at right angles to the direction of wave propagation, as seen in light waves and ripples on a water surface. The concept of traveling through a vacuum pertains more to the medium dependency of waves; longitudinal waves like sound cannot travel through a vacuum, but this property does not define the wave itself. Lastly, the visibility of the wave to the naked eye does not relate to its classification as longitudinal or transverse, since many types of waves exist outside the visible spectrum and both can be observable or non-observable depending on the wavelengths involved.