Consider an optimal risky portfolio $(=$ P$*)$ and a risk$-$free asset $(=$ F$).$ The optimal risky portfolio P$*$ consists of two stocks, S$1$ and S$2.$ Suppose all investors must choose a mix of P$*$ and F$.$ According to the separation property, the $\_\_\_\_\_$ must be the same for all investors.

Group of answer choices

expected return on the investor's mix of P$*$ and F

proportion of S$1$ and S$2$ in P$*$

proportion of S$1$ and S$2$ in the investor's mix of P$*$ and F

standard deviation of the investor's mix of P$*$ and F