Ion having highest hydration enthalpy among the given alkaline...

Smaller the size of the cation, more the hydration of ion, hence the more values of the hydration enthalpies. Now, as the down the group the size increases so the hydration enthalpies decreases, hence as the increasing order of the size of the ions is as follows:
${Be}^{2 +} < {Mg}^{2 +} < {Ca}^{2 +} < {Sr}^{2 +} < {Ba}^{2 +}$
and reverse of this order is true for the hydration enthalpies.
${Be}^{2 +} > {Mg}^{2 +} > {Ca}^{2 +} > {Sr}^{2 +} > {Ba}^{2 +}$