What aspect of electromagnetic radiation is explained by the Doppler effect?

Question

Im stuck between these three answers... I know it has to do with wave lengths of waves but i am still unsure of which one this would be.

Would it be the shift of spectrum lines toward or away from the standard positions? That's the one i think it is.

The only other two options I could see as being remotely possible are the shift in the apparent magnitude of the stars

and the mass of stars.

Answer 1

You're right.