Concept:Transverse and longitudinal waves differ in how the particles of the medium vibrate relative to the wave's direction of travel.Explanation:In a transverse wave, particles vibrate perpendicular to the direction of wave propagation.In a longitudinal wave, particles vibrate parallel to the direction of wave propagation.Thus, the essential difference is the direction of vibration of the particles of the medium.Answer:C. direction of vibration of the particles of the medium