Explanation

(c)
$AX_{2}$ is ionised as follows
$AX_{2}\ \ \ \ \ \ \ \ \rightleftharpoons \ \ \ A^{2 +}\ \ + \ \ \ 2X^{-}$
$S\ mol\ L^{- 1}$ $S$ $2S$
Solubility product of $AX_{2}$
$\left( K_{sp} \right) = \left\lbrack A^{2 +} \right\rbrack\left\lbrack X^{-} \right\rbrack^{2} = S \times (2S)^{2} = 4S^{3}$
$\therefore$ $K_{sp}\ of\ AX_{2} = 3.2 \times 10^{- 11}$
$\therefore$ $3.2 \times 10^{- 11} = 4S^{3}$
$S^{3} = 0.8 \times 10^{- 11} = 8 \times 10^{- 11}$
Solubility = $2 \times 10^{- 4}$ mol/L