Hydration enthalpy (ꕔH_Hyd) is the change in enthalpy when one mole of gaseous ion under a standard condition of 1 bar pressure dissolves in a sufficient amount of water to form an infinitely dilute solution (infinite dilution means a further addition of solute will not cause any heat change).

Hydration enthalpy is inversely related to the size of the ion, smaller the size of the ion greater is the hydration and hence greater is the hydration energy released.

Mn⁺² has the lowest hydration energy among the given ions as it has the largest size.

Ion	${\mathrm{\Delta H}}_{\mathrm{Hyd}.}^{\mathrm{o}}(\mathrm{kJ}/\mathrm{mole})$
${\mathrm{Cr}}^{2+}$	$-1925$
${\mathrm{Mn}}^{2+}$	$-1862$
${\mathrm{Fe}}^{2+}$	$-1998$
${\mathrm{Co}}^{2+}$	$-2079$