What type of wave requires a medium to propagate?

The correct choice is longitudinal wave because these waves inherently require a medium—such as air, water, or solid materials—through which to travel. Longitudinal waves are characterized by the oscillation of particles in the medium in the same direction as the wave travels. For example, sound waves are a common type of longitudinal wave, where compression and rarefaction of air particles propagate the sound through the medium.

In contrast, electromagnetic waves do not need a medium to travel. They can propagate through a vacuum, as they consist of oscillating electric and magnetic fields. Transverse waves, which include ripples on the surface of water or waves on a string, can also require a medium, but this is not a definitive aspect since they can propagate through elastic materials and also involve the movement of particles perpendicular to the direction of wave travel, as seen in certain contexts. Therefore, while some transverse waves do require a medium, the fundamental characteristic of requiring a medium for propagation is explicitly embodied in longitudinal waves.