Explanation

(a)
First ionisation energy increases from left to right across a period, but Mg has extra stability than Al, due to full-filled 3$s$-orbitals.
$Na_{11} = 1s^{2},2s^{2},2p^{6},3s^{1}$
$Mg_{12} = 1s^{2},2s^{2},2p^{6},3s^{2}$
$Al_{13} = 1s^{2},2s^{2},2p^{6},3s^{2},3p^{1}$
$Si_{14} = 1s^{2},2s^{2},2p^{6},3s^{2},3p^{2}$
The correct order of first ionisation potential is
$Na < Mg > Al < Si$