which refers to the change of pitch heard when the source of the sound approaches and then moves away from a…

Answer:

A. Doppler effect

Brief Explanations:

The Doppler effect describes the change in frequency (pitch) of a wave (like sound) when the source and observer have relative motion. When a sound - source approaches a stationary person, the pitch increases, and when it moves away, the pitch decreases. Volume is related to loudness, resonance is about a system vibrating at its natural frequency, and vibration is a general term for oscillatory motion, none of which describe the pitch - change in this context.