Question

A star’s light is shifted to the red part of the light spectrum.

Answer 1

If a star’s light is shifted to the red part of the light spectrum, that means
that the light waves we see when we look at that star are longer than
they SHOULD be ... longer than they were when they left the star.

Note:
The wavelengths are NOT "getting longer" while we sit there and look
at them. That doesn't happen. They ARE longer than they should be.

Right now, the only way we KNOW OF that can increase the wavelength
of light is if the source of the light is moving AWAY from us, and so we
mark that star down in our notebook, and next to it we write "This star is
moving away from us.". This is kind of what choice-C is trying to say.

The thing about this whole story that should blow our minds is this:

-- We observe a star or a galaxy.
-- The light we observe has wavelengths longer than they should be.
-- We say that the star or galaxy is moving away from us.

Now, my question to you is:
HOW do we know what the wavelengths SHOULD be ? ?
We only know what we see. How do we know what the
wavelength was when the light left the star or galaxy ?