Which d-block cation has the maximum hydration enthalpy?
Note that hydration energy is the energy released when water molecules are adsorbed on the surface of the cation. Hence the cation should have a high charge density, which means it should have a small size. So, the smaller, the better.

Here, $\ce{Ni}$ has the smallest atomic radii compared to all other options due to greater effective nuclear charge, so it's ionic radii would also follow the same trend. Hence, $\ce{Ni^2+}$ would have greater hydration enthalpy.