Sound waves cause longitudinal motion in air molecules

Steps:

• Sound waves are longitudinal, propagating as compressions and rarefactions in air.
• Molecules vibrate parallel to the wave's direction of travel.
• At point P, the molecule oscillates back and forth continuously at 200 Hz.
• Diagram A depicts this to-and-fro motion along the propagation axis.

Why A is correct:

• Longitudinal waves cause particle displacement parallel to propagation, per the definition of sound as a pressure wave.

Why the others are wrong:

• B shows transverse motion perpendicular to propagation, unlike sound waves.
• C shows circular or elliptical paths, not linear oscillation.
• D shows radial expansion or no directed motion, ignoring wave direction.

Final answer: A