Which type of wave has particles that oscillate along the direction of wave propagation?

Longitudinal waves are characterized by particle oscillation that occurs in the same direction as the wave is propagating. This means that as the wave travels through a medium, the individual particles of that medium move back and forth along the direction of the wave. An excellent example of this kind of wave is sound waves, where areas of compression and rarefaction move through the medium.

In contrast, transverse waves involve particle motion that is perpendicular to the direction of wave propagation, as seen in waves on a string or electromagnetic waves, which consist of oscillating electric and magnetic fields. Surface waves also involve a combination of both transverse and longitudinal motion, particularly in the context of water waves, where the water particles move in circular paths. Electromagnetic waves travel through a vacuum without requiring a medium and consist of oscillating electric and magnetic fields. Each of these other types does not fit the definition of particles oscillating along the wave's direction.