The first ionisation enthalpies of Na, Mg, Al and Si are in the order:
(i) Na < Mg > Al < Si
(ii) Na > Mg > Al > Si
(iii) Na < Mg < Al < Si
(iv) Na > Mg > Al < Si
The answer is the option (i) Na < Mg > Al < Si
Explanation: While moving across the periods, the increasing nuclear charge overpowers the shielding effect. Consequently, the valence electrons are held more and more tightly and thus; the ionisation enthalpy increases across the period. In case of Mg, the valence electron is in the s-orbital which is a stable gas configuration, and penetration effect of s-subshell is more as compared to aluminium (Al) in which the outermost electron is in p-subshell.