Explanation

(c)
$Cr(OH)_{3} \rightleftharpoons {Cr}^{3 +} + 3{OH}^{-}$
For precipitate to be dissolved,
$K_{sp} \leq \left\lbrack {Cr}^{3 +} \right\rbrack\left\lbrack {OH}^{-} \right\rbrack^{3}$
or $6 \times 10^{- 31} \leq \lbrack 0.1\rbrack\left\lbrack {OH}^{-} \right\rbrack^{3}$
$\therefore$ $\lbrack{OH}^{-}\rbrack \geq 1.79 \times 10^{- 10}$
So, $\lbrack H^{+}\rbrack \leq \frac{10^{- 14}}{1.79 \times 10^{- 10}} \leq 5.59 \times 10^{- 5}$
$pH \geq - \log{\left( 5.59 \times 10^{- 5} \right)\ \ \ or\ pH \geq 4.253}$