When the source of sound approaches the listener at the frequency or pitch of sound received by him is:

Explanation

The correct answer is that the frequency received by the listener is greater than the frequency of the sound produced by the source. This is due to the Doppler effect, which states that the observed frequency increases when the source of sound moves towards the observer. Option A is incorrect because it describes the situation when the source moves away from the observer. Option C is incorrect because it would imply no relative motion between the source and the observer. Option D is incorrect because the frequency cannot be zero unless the source stops emitting sound.