The ionization energy is the amount of the energy required to remove an electron from the valence orbital of the atom.

The ionization enthalpy depends on the atomic size, smaller the atomic radius the greater is the ionization enthalpy.

In option (i) the nuclear charge is on the last electron is very less so it will have less ionization energy compare to the option (ii).

(ii) and (iv) have exactly half-filled p-orbitals but (ii) is smaller in size.

Thus, [Ne]3s²3p³ has the highest ionization enthalpy.