Explanation

(c)
Potash alum is a double salt.
Potash alum, $K_{2}SO_{4}.Al_{x}\left( SO_{4} \right)_{3}.24H_{2}O$ (given)
Ions Al $SO_{4}$
Valency +3 -2
Therefore, $Al_{3}\left( SO_{4} \right)_{3}$ is compound of $Al^{3 +}$ and ${SO}_{4}^{2 -}$.
On comparing, x=2
Hence, formula of potash alum is
= $K_{2}SO_{4}.Al_{2}\left( SO_{4} \right)_{3}.24H_{2}O$