Martinez is looking at the positions of the four stocks (a, b, c, and d in blue) that he is considering on the Security Market Line (SML) as below. When he is a risk-averse and highly rational investor, which stock is he supposed to purchase among the four stocks?
 

Asset expected return (E(R;)) E(RM) R PM=1.0 a d C E(RM) - R Asset beta (B)