Explanation

(a)
As we go down the group in Periodic Table, atomic size increases, force of attraction for the added electron decreases, hence electron gain enthalpy decreases.
$X(g) + e^{-} \rightarrow X^{-}(g)$
Actual order, $Cl > F > Br > I$
The fact that fluorine has a less electron gain enthalpy than chlorine seems to be due to the relatively greater effectiveness of 2$p$-electron in the small F-atom to repel the additional electron entering the atom than do 3$p$-electrons in the larger Cl-atom.