As we move down in a group, electron gain enthalpy becomes less negative because the size of the atom increases and the distance of added electron from the nucleus increases. Negative electron gain enthalpy of F is less than Cl. This is due to the fact that when an electron is added to F, the added electron goes to the smaller n=2 energy level and experiences significant repulsion from the other electrons present in this level. In Cl, the electron goes to the larger n=3 energy level and consequently occupies a larger region of space leading to much less electron-electron repulsion. So the correct order is Cl>F>Br>I