Choosing the optimal risky portfolio?

In this diagram, why is P the optimal risky portfolio? Y is acheivable and provides a higher return. Why isn’t Y the optimal risky portfolio?

http://imgur.com/xF9rqOH

Tangency of efficient frontier and CAL is optimal risky portfolio. The meaning of optimal risk portfolio is in fact there is not better return for given unit of risk. Y is acheviable and provides only better return but also is more risky than portfolio P. You cannot follow only better return with don’t consider the risk.