Explanation

(d)
Potash alum is double salt. Its chemical composition is
$K_{2}SO_{4}.Al_{2}\left( SO_{4} \right)_{3}.24H_{2}O$
$K_{2}SO_{4}.Al_{2}\left( SO_{4} \right)_{3}.24H_{2}O \rightarrow 2K^{+} + 2Al^{3 +} + 4SO_{4}^{2 -} + 24H_{2}O$
$\therefore$ It gives three types of ions on dissociation
$\ \ \ \ \ \ \ \ \ \ \ \ K^{+},Al^{3 +}$and $SO_{4}^{2 -}$