What type of wave motion does a sound wave exhibit when it moves back and forth in the direction of propagation?

A sound wave exhibits longitudinal wave motion when it moves back and forth in the direction of propagation. In longitudinal waves, the particles of the medium through which the sound travels vibrate in the same direction as the wave itself. This results in alternating areas of compressions (where particles are close together) and rarefactions (where particles are spread apart) as the wave moves through the medium.

In contrast, transverse waves have particle motion that is perpendicular to the direction of wave propagation. An example of this would be waves on a string or water waves where the movement occurs up and down while the energy travels horizontally.

The terms refractive and mechanical describe other aspects of wave behavior. Refractive refers to the bending of waves as they pass from one medium to another, while mechanical pertains to waves that require a medium to travel, such as sound waves, but does not specifically describe the pattern of particle motion in sound waves.

Thus, the correct choice reflects the nature of how sound waves move through mediums, specifically demonstrating longitudinal motion as the particles oscillate in the same direction as the advancing wave.