Stock B is undervalued because it’s under SML, and it is higher risk because it’s beta is higher than one. It’s expected return is too low, so sell it. Once people begin to sell it, it will drive down the price and correct itself in terms of equilibrium.
Stock A is undervalued because it’s under SML, it’s relatively low risk as it’s below beta but that means it’s not very profitable.
Stock C is undervalued, you get a higher return that expected therefore people will buy it and eventually drive up the price and reach equilibrium.