(b): On moving down the group in halogens, the electron gain enthalpy becomes less negative. This is due to the fact that the atoms of these elements have only one electron less than stable noble gas configurations. However, the negative electron gain enthalpy of fluorine is less than that of chlorine. Because of small size, interelectronic repulsions are stronger in relatively small $2 p$ orbitals of fluorine and there is not much attraction of the nucleus on the incoming electron, due to higher electron density. So, chlorine has highest value of electron gain enthalpy.
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