Hydration enthalpy is the amount of energy released when one mole of ions are dissolve in - excess of water. Hydration enthalpy increases as the ionic size decreases and its charge increases. Na+and Mg2+ belong to third period and therefore have smaller size than K+and Ca2+ of fourth period. Mg2+ is smaller than Na+and also has higher charge than Na+, therefore Mg2+ will have highest hydration enthalpy.