The answer I had got was C x
That's called the "red shift" of the star's spectrum, because red is the lowest frequency/longest wavelength in visible light.
Answer:

Explanation:
The force between charges, 
Positive charge, 
Negative charge, 
We need to find the distance between charges. The force of attraction between charges is given by :

Where
r is the distance between charges

So, the distance between charges is
.