In your role as an investment analyst, you are evaluating the risk and stock valuation of a potential investment. Through your analysis of the security market line equation in accordance with the investment, you note that the stock tends to outperform in up markets and underperform in down markets. This would be most likely in direct relation to which component of security market line?

a$.$ Expected market return

b$.$ Risk$-$free rate

c$.$ Discount rate

d$.$ Beta $(\backslash$beta $)$