The aqueous solution of potash alum $[K_{2}S{O}_4.A{l}_2(S{O}_4{)}_3.24H_{2}O]$ is acidic due to:

The correct option is C hydrolysis of $A{l}^{3+}$
Potash alum is $KAl{\left(S{O}_4\right)}_2$. In aqueous medium due to hydrolysis of ${Al}^{3+}$, it is acidic than nature.

When ${Al}^{3+}$ ions are solvated by water molecules their high pulled electron density towards them, making electron density between O-H in solvating water molecules become less. Protons are therefore more likely to be abstracted by water.