Which refers to the change of pitch heard when the source of the sound approaches and then moves away from a stationary person? Doppler effect volume resonance effect vibration

The Doppler effect is a change in the frequency of sound waves that occurs when the source of the sound waves is moving relative to a stationary listener. As the source of sound waves approaches a listener, the sound waves get closer together, increasing their frequency and the pitch of the sound.HOPE IT HELPS THANK YOU

Ithink the answer is d

Ithink it is a. but i'm not 100 percent sure.