(A) 3s2→Mg
(B) 3s23p1→Al
(C) 3s23p3→P
(D) 3s23p4→S
Al has one unpaired electron in its highest energy orbital (3p), and Mg′s highest energy orbital (3s) has the paired electrons.
It is energetically favourable for all the electrons in an orbital to be paired, which means that breaking up this pair would require more energy.
Compared to 16th group, 15 group have high ionisation energy. It is because of the stable half filled configuration of he 15th group elements.
P−3s23p3
S−3s23p4

Therefore, P has more ionisation energy than S.
Hence, the correct order of first ionization enthalpy is B<A<D<C