This question tests you on your understanding of the
If the observer or the source is standing still, or not moving, its speed is taken to be
If the observer is moving toward the source, the (+) sign is used, if observer is moving away, the (-) sign is used
If the source is moving toward the observer, the (-) sign is used, if source is moving away, the (+) sign is used.
The equation to use to solve this problem is,
This answer actually makes sense because if you have even stood still and heard an ambulance or firetruck approaching you, the frequency of the sound which you observe of the trucks seems to be higher (its pitch). This is exactly what is going on in this problem.