The Capital Asset Pricing Model $($CAPM$)$ is:

A$)$ None of these answers are correct

B$)$ Based on the proposition that a stock held in a well diversified portfolio is less risk than the same stock held in isolation.

C$)$ Based on the proposition that a stock held in a well diversified portfolio is more risky than the same stock held in isolation.

D$)$ Based on proposition that a stock held in a well$-$diversified portfolio has a standard deviation that is smaller than that of the entire portfolio.

Choose one answer from above.