Which of the following characteristics is true about longitudinal waves?

Longitudinal waves are characterized by the direction of their vibrations, which occur in parallel to the direction of wave propagation. This means that as the wave travels, the particles of the medium through which the wave is moving oscillate back and forth in the same direction as the wave itself.

For instance, in a sound wave, the air particles compress and then rarefy as the wave moves through the medium. This behavior is specific to longitudinal waves, distinguishing them from transverse waves, where the vibrations occur at right angles to the direction of wave travel.

The other characteristics mentioned are not applicable to longitudinal waves. They do not have crests and troughs which are indicative of transverse waves; they do not inherently travel faster than transverse waves as the speed of wave propagation depends on various factors including the medium; and they do not require a vacuum to propagate, as longitudinal waves can travel through solids, liquids, and gases.