Question

When a horn is moving away from an observer the sound waves from it will pass by the observer____ frequently, causing the pitch to____

Answer 1

The answer: less frequency, causing the pitch to be lowered. 

This is known as the Doppler effect. When you have an object that is producing sound while moving, the side that it is moving towards to will have waves that will become a bit more squished together, while the other side that it is moving away from will have waves that are spaced a bit more apart from each other. The only determining factor in this is the magnitude of the speed, so the faster the object is moving towards or away from you, then the more severe pitch change the object will have.
In this case, it's just asking in general what would happen if the horn was moving away from you while it was giving off sound waves. Therefore, since it is moving away from you, its sound waves are a bit further apart, resulting in a lowered frequency and pitch.