D) sound is molecules bumping into one another.
Explanation:
Sound waves are longitudinal waves generated by the vibrations of the molecules of a medium. These oscillations occur in a direction parallel to the direction of propagation of the wave (longitudinal wave), so basically the wave is transmitted by the collisions between the molecules of the medium, and therefore D. is the correct answer. Morevoer, these oscillations create regions of higher density of molecules (called compressions) and regions of lower density of molecules (called rarefactions).
