Correct Response: A. The Doppler effect is an apparent shift in the frequency of a wave as experienced by an observer when the source is in motion relative to the observer. The frequency is experienced as shifting higher when the source is approaching the observer, and it is experienced as shifting lower when the source is moving away from the observer. Hence, the ambulance's siren would change from a high pitch as it is approaching to a low pitch as it is receding (A). The opposite (B) would not be the case. The amplitude (C and D) of the experienced sound wave would only depend on the distance of the ambulance, and not on whether it was approaching or receding.
