Question

The Doppler Effect is produced.... *

Answer 1

C. Because as either one or both of the objects receiving or producing the sound has to move to create the doppler effect.

Answer 2

The Doppler effect is observed when the distance between
the source and the observer is changing.

You have to say it that way, because 'motion' isn't enough.
It has to be motion along the line between them.

For example, if the source is whizzing in a circle around the
observer, or the observer around the source, then the distance
between them isn't changing, and there's no Doppler effect.