\({ }_8 O \rightarrow 1 s ^2 \quad 2 s ^2 \quad 2 p ^4\)

\({ }_9 F \rightarrow 1 s ^2 \quad 2 s ^2 \quad 2 p ^5\)

\({ }_7 N \rightarrow 1 s ^2 \quad 2 s ^2 \quad 2 p ^3\)

\({ }_6 C \rightarrow 1 s ^2 \quad 2 s ^2 \quad 2 p ^2\)

\(2 nd\,\,IE \rightarrow O ^{-} \rightarrow 1 s ^2 \quad 2 s ^2 \quad 2 p ^3\)

\(F ^{-} \rightarrow 1 s ^2 \quad 2 s ^2 \quad 2 p ^4\)

\(N ^{-} \rightarrow 1 s ^2 \quad 2 s ^2 \quad 2 p ^2\)

\(C ^{-} \longrightarrow 1 s ^2 \quad 2 s ^2 \quad 2 p ^1\)

Oxygen having half filled configuration is most stable and will require high energy to remove an \(e ^{-}\). Followed by fluorine being the most electronegative element, then followed by \(N\) which is less electronegative than \(F\) but more than \(C\).

\(\therefore\) Order of second ionization enthalpy is \(O\, >\, F\, >\, N\, >\, C\)