The correct option is : (b) O < S < F < Cl
Explanation:
Cl atom has the highest electron affinity in the periodic table. F being a member of group 17 has higher electron gain enthalpy than S which belongs to group 16. This in turn is higher than the electron affinity of O atom. Thus,
Cl > F > S > O
It is worth noting that the electron gain enthalpy of oxygen and fluorine, the members of the second period have less negative values than the elements sulphur and chlorine of the third period.
This is due to small size of the atoms of oxygen and fluorine. As a result, there is a strong inter-electronic repulsion when extra electron is added to these atoms, i.e., electron density is high and the addition of an extra electron is not easy.
