Explanation

(d)
Buffer capacity of an acidic buffer is maximum when the ratio of $HA\ to\ A^{-}$ is unity.
Since, pH of acidic buffer = $pK_{a} + \log\frac{\lbrack A^{-}\rbrack}{\lbrack HA\rbrack}$
For maximum buffer capacity, $\left\lbrack A^{-} \right\rbrack = \lbrack HA\rbrack$
$\therefore$ $pH = pK_{a}$