AL physics 2004 mc(13)

The answer is D.

A. Because of resonance, the observed loudness is greater than just with the fork itself.

B. The incident sound (longitudinal) wave superimposes with the reflected wave, and resonance occurs when a stationary wave is formed.

C. Particles at the closed end of the tube do no vibrate, and hence the pressure at that point fluctuates by the maximum extent (pressure antinode)

D. It is not necessary for their frequencies to be the same. There can be first overtune, second overtone, e.t.c.