b
Electron gain enthalpy, generally, increases in a period from left to right and decreases in a group on moving downwards. However, members of ll period have somewhat higher electron gain enthalpy as compared to the corresponding members of second period, because of their small size.

\(O\) and \(S\) belong to \(VI A ( 16)\) group and \(Cl\) and \(F\) belong to \(VII\) \(\mathrm{A}(17)\) group. Thus, the electron gain enthalpy of \(\mathrm{Cl}\) and \(\mathrm{F}\) is higher as compared to \(O\) and \(S\).

\(\mathrm{Cl}\) and \(\mathrm{F}>0\) and \(\mathrm{S}\)

Between \(\mathrm{Cl}\) and \(\mathrm{F}, \mathrm{Cl}\) has higher electron gain enthalpy then the \(F,\) since the incoming electron experiences a greater force of repulsion because of small size of \(F-\)atom. Similar is true in case of \(O\) and \(S\), i.e. the electron gain enthalpy of \(S\) is higher as compared to \(O\) due to its small size. Thus, the correct order of electron gain enthalpy of given elements is

\(\mathrm{O}<\mathrm{S}<\mathrm{F}<\mathrm{Cl}\)