Question

Sam is walking through the park. He hears a police car coming down the street toward him. What happens to the sound of the siren as the police car approaches and then passes Sam?
Sound waves from the siren remain constant. As the police car drives past Sam, the sound waves decrease in frequency.
The frequency of sound waves from the siren increase. As the police car continues past Sam, the sound waves decrease in frequency.
The frequency of sound waves from the siren decrease. As the police car drives past Sam, the sound waves increase in frequency.
The frequency of the sounds waves from the siren increase and continues to increase as the police car passes and drives away from Sam.

Answer 1

I believe it is the first one.
This is because of the Doppler Effect. 
The Doppler Effect is when the frequency of the wave changes for an observer moving relative to the wave source.
Basically, as the police car moves away from you, the distance between you and the car increases, which makes the sound waves spread out. 

Answer 2

Answer:

The correct answer is "The frequency of sound waves from the siren increase. As the police car continues past Sam, the sound waves decrease in frequency."

Explanation:

The correct answer is "The frequency of sound waves from the siren increase. As the police car continues past Sam, the sound waves decrease in frequency."

This is due to the effect called "Doppler Effect".

The Doppler effect is defined as the change in apparent frequency of a wave produced by the relative movement of the source with respect to its observer. In other words, this effect is the change in the perceived frequency of any wave movement when the sender and the receiver, or observer, move relative to each other.

According to this effect, the frequency perceived by the receiver will increase when the receiver and sender increase their separation distance and will decrease whenever the separation distance between them is reduced. As mentioned, then when the police car approaches Sam, the wave "compresses", that is, the wavelength is short, and the frequency is high. On the other hand, when the police car moves away, the wave "decompresses", that is, the wavelength is long and the frequency low.