Which of the following equations represents the relation between solubility and solubility product for salt BA$_3$?

Calculate the pH of buffer solution containing 0.027 M weak acid and 0.054 M of its salt with strong base if $\mathrm{pK}_{\mathrm{a}}$ is 4.2.

Calculate $\left[\mathrm{H}_3 \mathrm{O}^{+}\right]$ of a monobasic acid if it is $0.04 \%$ dissociated in 0.05 M solution.

Calculate the solubility product of sparingly soluble salt BA at 300 K if its solubility is $9.1 \times 10^{-3} \mathrm{moldm}^{-3}$ at same temperature.