Electron gain enthalpy, generally, increases in a period from left to right and decreases in a group on moving downwards. However, members of III period have samewhat higher electron gain enthalpy as compared to the coressponding members of second period, because of their small size. O and S belong to VI‌A(16) group and Cl and F belong to VII A(17) group. Thus, the electron gain enthalpy of Cl and F is higher as compared to O and S. Cl and F>O and S Between Cl and F,Cl has higher electron gain enthalpy as in F, 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 ie, 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 O<S<F<Cl