The correct answer is option 4, the Doppler effect. The pitch of a sound is determined by the frequency at which the sound waves reach our ears. The Doppler effect describes the change in frequency (and thus pitch) of a wave when the source of the wave or the observer of the wave is in motion relative to each other.

In the case of the fast moving train, as it approaches an observer, the sound waves it produces are compressed or squished together due to the relative motion between the observer and the train. This compression increases the frequency of the waves, making the pitch of the whistle sound higher.

When the train passes by and moves away from the observer, the sound waves are stretched out, resulting in a lower frequency and a lower pitch.

The Doppler effect is not to be confused with the Compton effect (option 1), Raman effect (option 2), or Kerr effect (option 3), which are unrelated phenomena in physics and do not explain the change in pitch observed in this scenario.