How does electromagnetic spectrum related to expanding universe

(This is not biology)

Some of the proof that the Universe is still expanding is the Doppler Effect, which consists of : when radiation is emitted, its frequence is higher the closer it is to the object receiving that radiation. So when explaining the universe expansion cientists base on the fact that the radiation emitted by the planets are reducing their frequences gradually.

I tried my best, I hope this helps!

i believe the correct answer is d