Explanation

(b)
Electron affinity is defined as, “The energy released when an extra electron is added to a neutral gaseous atom.”
Electron affinity of F=332.6 kJ/mol
Electron affinity of Cl=348.5 kJ/mol
Electron affinity of S=200.7 kJ/mol
Electron affinity of O=140.9 kJ/mol
Highest electron affinity among fluorine, chlorine, sulphur and oxygen, is of chlorine.
The low value of electron affinity of fluorine than chlorine is probably due to small size of fluorine atom i.e., electron density is high which hinders the addition of an extra electron.