Answer: The correct answer is A.
Step-by-step explanation:
When there is a relative motion between the source and the observer then there will be change in the frequency or wavelength of the wave. This phenomenon is called Doppler effect.
For example, when the source and the observer are moving away from other then the frequency decreases in this case.
In the given problem, the person is in the back of the ambulance. In this case, the person in the back of the ambulance will listen a lower frequency of the wave. There will be longer wavelength.
Therefore, the correct answer is A.