In a longitudinal wave, the medium's particles move parallel to the direction of travel.

• A. Perpendicular
• B. Circular
• C. Random
• D. Parallel

Answer: (D)

Longitudinal waves carry energy as particles move parallel to the direction the wave travels. As the wave moves, the particles oscillate back and forth along the same line, creating compressions and rarefactions that propagate through the medium. This parallel motion is what distinguishes longitudinal waves from transverse waves, where the particle motion is perpendicular to the direction of travel; circular motion would imply the particles move in circles, not back and forth along the travel direction, and random motion has no organized pattern to carry the wave. So the motion along the same line as the wave’s travel best describes a longitudinal wave.