Mg cannot exhibit +3 oxidation state, because it…

Mg cannot exhibit +3 oxidation state, because it ${\mathrm{IE}}_3\gg {\mathrm{IE}}_2$.

Two valence shell electrons are simply removed ($3s^2$) in the +2 oxidation state, making it simple to obtain $M{g}^{2+}$. An additional electron from the inner 2$p$ shell with inert configuration of neon must be removed in the +3 oxidation state ($M{g}^{3+}$). Since a lot more energy is needed for this, the third ionization energy is greater to the second ionization energy.

Hence, option C is correct.