Ion having highest hydration enthalpy among the given alkaline earth metal ions is :

<p>Hydration enthalpy is the amount of energy released when one mole of gaseous ions is completely surrounded by water molecules. The process is exothermic, hence the enthalpy change is negative. </p> <p>Generally, smaller ions and ions with higher charge have larger hydration enthalpies due to the stronger attraction to the water molecules. This is because the hydration enthalpy depends on the charge density of the ion. </p> <p>Among the given options, $\mathrm{Be}^{2+}$ is the smallest ion and it has the same charge as the other ions. Hence, it has the highest charge density and will have the highest hydration enthalpy. </p>