The CORRECT order of first ionisation enthalpy is:

The ionization energy of the elements increases from left to right in the periodic table. The first ionization of magnesium is greater than that of aluminium because magnesium has a stable complete electronic configuration $1{s}^{2}2{s}^{2}2{p}^{6}3{s}^{2}$, so it requires more energy to remove the first electron from it, whereas aluminium has electronic configuration $1{s}^{2}2{s}^{2}2{p}^{6}3{s}^{2}3{p}^{1}$ which need lesser energy than that of magnesium. Similarly, phosphorus has more ionisation energy than sulphur due to the half filled electronic configuration in the phosphorus.

$MgAlPS \to IE . order ⇒ Al < Mg < S < P$