The correct order of hydration enthalpies is (A) K ^ + (B) Rb ^ +...

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

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

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: ${Be}^{2 +} > {Mg}^{2 +} > {Ca}^{2 +} > {Sr}^{2 +} > {Ba}^{2 +}$

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

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