Which halogen has the highest value of negative electron gain enthalpy ?

Correct Answer - B
Halogen have highest negative electron gain enthalpy in their respective periods, because they have only one electron less than stable noble gas configurations. As a result, they readily accept one electron to acquire the noble gas configuration. Electron gain enthalpy becomes less negative on moving from chlorine to iodine. However, negative electron gain enthalpy of fluorine is less than that of chlorine due to small sized of fluorine in the relatively small `2p`-orbitals. Hence, incoming electron experience less attraction from the nuclues.
`:.` Chlorine has the highest value of electron gain enthalpy.