Why does the sound of something moving away from you seem to change to a lower and lower pitch? a) as the source moves away, the increased distance creates more interference; so the pitch drops. b) as the source moves away, the frequency of the sound waves is being compressed, so the pitch drops. c) as the source moves away, the sound waves speed up to reach the observer; this causes the pitch to drop. d) as the source moves away, the sound waves stretches out relative to the person standing still; so the pitch drops.

The sound of something moving away from you does NOT seem
to change to a lower and lower pitch.  It simply has a lower pitch
than the sound that was actually emitted.  HOW MUCH lower depends
on the speed with which the source is moving away from you.

As the source moves away, the sound waves "stretch out" relative to
the person standing still; so the pitch drops.  (D)

By the way ... it doesn't matter whether it's the source or the listener moving away, or the listener or the source standing still.  As long as
the distance between them is increasing, the listener hears a sound
with a pitch that's lower than it should be.