What characterizes a longitudinal wave?

A longitudinal wave is defined by the way it moves through a medium. In this type of wave, the particles of the medium oscillate back and forth in a direction that is parallel to the direction of the wave's travel. This creates areas of compression and rarefaction, where the particles are closer together and farther apart, respectively. This characteristic distinguishes longitudinal waves from transverse waves, where particles move perpendicular to the direction of wave propagation. Understanding this distinction is crucial for grasping various wave phenomena encountered in fields such as physics and engineering.

The other options present different wave characteristics that do not apply to longitudinal waves. For instance, transverse waves are described by option B, which indicates movement perpendicular to the wave's direction, and electromagnetic waves would relate to option C, as they do not require a medium. Finally, option D refers to properties of waves in general but is not specific to longitudinal waves.