The change in pitch of a train's horn as it passes while you are standing still can be explained by
a. the doppler effect.
b. ultrasonic vibrations.
c. sympathetic vibration.
d. resonance.

The change in pitch of a train's horn as it passes while you are standing still can be explained by the Doppler effect. The correct option among all the options given in the question is option "A". When the sound source from the horn of the train comes closer, the sound seems to come closer each time a sound wave gets emitted. So it comes closer to us in a bunched form. The reverse happens when the train has just passed with each emitted sound going further and further away. The frequency of the sound seems to get lower as the train moves further away.