In the diagram below, a stationary source located at point s produces sound having a constant frequency of 512 hertz. observer a, 50 meters to the left of s, hears a frequency of 512 hertz. observer b, 100.0 meters to the right of s, hears a frequency lower than 512 hertz. a) observer a is stationary, and observer b is moving away from point s. b) observer a is stationary, and observer b is moving toward point s. c) observer a is moving away from point s, and observer b is stationary. d) observer a is moving toward point s, and observer b is stationary.

This has to do with Doppler effect. If the source or the observer moves, they the observer will hear a different frequency. Since the source is stationary, we only need to know whether the observer is moving.

Since Observer A does not hear a change in frequency, he a must be stationary. So we can ignore C and D

Since Observer B hears a different frequency, Observer B must be moving. Here's the tricky part. We need to find whether B is moving towards or away the source. Since he hears a lower frequency, he is walking away from the source. So the answer is A.

You can conform with the Doppler equation.

A