In Doppler's effect, when a source of sound moves away from a stationary listener then the apparent frequency

Explanation

The Doppler effect describes the change in frequency of a wave in relation to an observer who is moving relative to the wave source. When a source of sound moves away from a stationary listener, the sound waves are spread over a larger distance, causing the wavelengths to increase and the frequency to decrease. Therefore, the apparent frequency perceived by the listener decreases. Option B is correct. Option A is incorrect because the frequency increases only when the source approaches the listener. Option C is incorrect because the frequency does not become zero unless the sound source ceases emission. Option D is incorrect because the frequency changes due to the relative motion between the source and the listener.