The correct option is B [Ba2+][SO2−4]
For the reaction BaSO4(s)⇌Ba2+(aq)+SO2−4(aq), the expression for the solubility product Ksp is [Ba2+][SO2−4]. It is the product of the molar concentrations of cations and anions. The concentration of each ion is raised to the power of its coefficient in the balanced equilibrium equation.