D) distance of source from listener
Doppler effect is the apparent change observed by an observer in the frequency of a wave (such as light or sound) whenever the source of the wave is moving relative to the observer.
The figure above shows Doppler effect for Sound. As the source of sound moves towards the listener the the pitch appears higher and as the source moves away the pitch appears to decrease.
the relationship between heard frequency
From the equation we see that it is independent of the distance of source from listener.