In which of the following situations is the Doppler Effect absent? The source and the observer are moving towards each other.

The observer is moving toward the source.

The source is moving away from the observer.

The source and the observer are both moving with the same velocity.

Correct option is

C

The listener is moving towards the source of sound

The Doppler effect (or the Doppler shift) is the change in frequency or wavelength of a wave (or another periodic event) for an observer moving relative to its source.

So if both listener and source of sound are stationary or are moving with the same velocity that is relative velocity is zero, there won't be any change in frequency.

Thus, in this case, the effect will arise when a listener is moving towards the source of the sound.