Down the group, electron gain enthalpies become less and less negative with the increase in the size of the halogen. However, electron gain enthalpy of $\mathrm{F}$ is less negative than $\mathrm{Cl}$. This is due to small size of F-atom. As a result, there are strong inter electronic repulsions in the relatively small $2 p$-orbitals of $\mathrm{F}$ and thus incoming electrons experience much less attraction. Thus, the correct order is $\mathrm{Cl}>\mathrm{F}>\mathrm{Br}>\mathrm{I}$.