a,
$Na: 1s^2$ $2s^2$ $2p^6$ $3s^1$
$Mg: 1s^2$ $2s^2$ $2p^6$ $3s^2$
$S: 1s^2$ $2s^2$ $2p^6$ $3s^2$ $3p^4$
$Cl: 1s^2$ $2s^2$ $2p^6$ $3s^2$ $3p^5$
b,
$+) Na\to Na^++e$
$Na^+: 1s^2$ $2s^2$ $2p^6$
$+) Mg\to Mg^{2+}+2e$
$Mg^{2+}: 1s^2$ $2s^2$ $2p^6$
$+) S+2e\to S^{2-}$
$S^{2-}: 1s^2$ $2s^2$ $2p^6$ $3s^2$ $3p^6$
$+) Cl+e\to Cl^-$
$Cl^-: 1s^2$ $2s^2$ $2p^6$ $3s^2$ $3p^6$