Longitudinal waves involve particle vibration parallel to propagation direction

Steps:

• Define longitudinal waves: oscillations where medium particles move parallel to wave travel.
• Identify sound waves: compressions and rarefactions cause parallel particle motion.
• Contrast with transverse waves: perpendicular particle motion, common in electromagnetic and surface waves.
• Evaluate options against definitions to select the match.

Why A is correct:

• Sound waves propagate via alternating high- and low-pressure regions, with water particles displacing parallel to the wave direction, per the definition of longitudinal waves.

Why the others are wrong:

• B: Ultraviolet waves are electromagnetic, with electric/magnetic fields oscillating perpendicular to propagation (transverse).
• C: Surface water waves involve particles moving in circles or ellipses perpendicular to the main propagation (transverse).
• D: X-rays are electromagnetic radiation, propagating transversely in vacuum without medium particle involvement.

Final answer: A