The correct order of hydration enthalpies is

(A) ${\mathrm{K}}^{+}$
(B) ${\mathrm{Rb}}^{+}$
(C) ${\mathrm{Mg}}^{2+}$
(D) ${\mathrm{Cs}}^{+}$
(E) ${\mathrm{Ca}}^{2+}$

Choose the correct answer from the options given below:

The hydration enthalpy of alkali metal ions decreases with an increase in ionic sizes.

So, the order is: ${\mathrm{Li}}^{+} > {\mathrm{Na}}^{+} > {\mathrm{K}}^{+ }> {\mathrm{Rb}}^{+} > {\mathrm{Cs}}^{+}$

Like alkali metal ions, the hydration enthalpies of alkaline earth metal ions decrease with an increase in ionic size down the group.

So, the order is: ${\mathrm{Be}}^{2+} > {\mathrm{Mg}}^{2+} > {\mathrm{Ca}}^{2+} > {\mathrm{Sr}}^{2+} > {\mathrm{Ba}}^{2+}$ 

The hydration enthalpies of alkaline earth metal ions are larger than those of alkali metal ions. 

So, the correct order is: ${\mathrm{Mg}}^{2+} > {\mathrm{Ca}}^{2+} > {\mathrm{K}}^{+} > {\mathrm{Rb}}^{+} > {\mathrm{Cs}}^{+}$