Which of the following is an example of double salt.

Salt which on complete ionization gives two different simple cations and a simple anion or two different anions and a simple cation is a double salt.

$$\begin{gathered} \mathrm{KCl}.{\mathrm{MgCl}}_2.6{\mathrm{H}}_2\mathrm{O}\to {\mathrm{K}}^{+}+{\mathrm{Mg}}^{+2}+3{\mathrm{Cl}}^{-} \\ {\mathrm{FeSO}}_4.{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4.6{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Fe}}^{+2}+2{\mathrm{NH}}_4^{+}+2{\mathrm{SO}}_4^{-2} \\ {\mathrm{K}}_2{\mathrm{SO}}_4.{\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3.24{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{K}}^{+}+2{\mathrm{Al}}^{+3}+3{\mathrm{SO}}_4^{-2} \end{gathered}$$