Sound waves are longitudinal with alternating compressions and rarefactions

Steps:

• Sound waves involve particle vibrations parallel to the wave direction, classifying them as longitudinal.
• Longitudinal waves feature compressions (high particle density) and rarefactions (low particle density).
• The diagram shows molecule positions; X marks a region of spaced-out molecules, indicating low density.
• Thus, X represents a rarefaction in a longitudinal sound wave.

Why A is correct:

• Sound waves are longitudinal by definition, as they propagate via parallel molecular oscillations; rarefactions occur where molecules are spread apart, matching the low-density region at X.

Why the others are wrong:

• B: Compression involves high molecular density, opposite to the spaced-out molecules at X.
• C: Transverse waves have perpendicular particle motion, but sound requires parallel motion through substances.
• D: Transverse waves do not produce rarefactions in sound propagation, which is inherently longitudinal.

Final answer: A