Which type of wave is characterized by particle displacement parallel to the direction of wave travel?

The correct choice is longitudinal wave. This type of wave is distinctive because the particles of the medium through which the wave travels move in a direction that is parallel to the direction of the wave's energy transfer. In simpler terms, as the wave propagates, the particles compress and then rarefied (or spread apart) in the same direction the wave is moving. This behavior is fundamental in sound waves, where regions of compression and rarefaction occur, allowing sound to travel through various media.

In contrast, transverse waves have particle displacement that occurs perpendicular to the direction of wave travel. This is evident in waves on a string or electromagnetic waves, where particles move up and down while the wave moves forward. Standing waves are a specific case of wave interference that results in stationary patterns, while surface waves combine characteristics of both transverse and longitudinal waves, often seen in water where the movement is both up and down and forward motion occurs. Understanding these characteristics helps clarify why a longitudinal wave is defined by its parallel displacement in relation to wave propagation.