Choice-C (the 3rd one on the list) is sorta kinda on the right track, and it's
the answer that the question is fishing for, but it has a glaring error.
An example of the Doppler shift:
If a star is moving toward us, then the frequency of the light we see
from the star appears higher than it was when it left the star.
The frequency of the light does NOT "increase as the star moves toward us".
It's constant and it doesn't change. It just appears higher than it should be.
The whole trick to this kind of observation is this: When we receive light from
a star and measure its frequency, HOW do we know what it SHOULD BE ? ! ?