Which of the following gives the best definition of the CAPM approach?

a)Unsystematic risk, as measured by beta, is the only factor affecting the level of return required on a share for a completely diversified investor.

b)Unsystematic risk, as measured by beta, is one of the key factors affecting the level of return required on a share for a completely diversified investor.

c)Systematic risk, as measured by beta, is the only factor affecting the level of return required on a share for a completely diversified investor.

d)Systematic risk, as measured by beta, is one of the key factors affecting the level of return required on a share for a completely diversified investor.

I can't choose between C or D. It looks quite similar

Please elaborate more on the answer