The electronic configuration of Na,Ne,Mg and Al after the removal of an electron is as Na+:1s2,2s2,2p6 Ne+:1s2,2s2,2p5 Mg+:1s2,2s2,2p6,3s1 Al+:1s2,2s2,2p6,3s2 The ionisation energy decreases as the size of the atom/ion increases. Further ionisation energy increases with increase in nuclear charge. Since, Na+ gives a stable electronic configuration hence its IE2 is highest among the given species and therefore the correct order of their IE2 is Mg<Al<Ne<Na