Can someone help me with this question?

According to portfolio theory all of the following statements are true when there is a risk less asset except:

a) the optimal risky asset portfolio should should always be mixed with investments in risk less bonds

b) the optimal risky asset portfolio minimizes the risk per unit of return risk premium

c) the optimal risky asset should be bought even by investors who even do not want to bear much risk

d) the optimal risky asset portfolio minimizes the sharpe measure