Choose the correct order of Ist ionisation energy.

Nuclear charge rises, atomic size falls, and ionisation enthalpy rises as we travel through the period. For $\mathrm{Al}\left(3{\mathrm{s}}^{2}3{\mathrm{p}}^1\right)$ it is necessary to remove an electron from a partially filled 3p orbital. On the other hand, $\mathrm{Mg}\left(3{\mathrm{s}}^2\right)$ in the electron in the stable, fully occupied 3s orbital needs to be removed. More energy is needed to remove an electron from a stable, fully filled orbital than from a partially filled orbital. As a result, the ionisation enthalpy for magnesium is higher than that of aluminium. Hence,  the correct order of Ist ionisation energy is $\mathrm{Mg}>\mathrm{Al}$.