$O$ and $S$ belong to $VI A ( 16)$ group and $Cl$ and $F$ belong to $VII$ $\mathrm{A}(17)$ group. Thus, the electron gain enthalpy of $\mathrm{Cl}$ and $\mathrm{F}$ is higher as compared to $O$ and $S$.

$\mathrm{Cl}$ and $\mathrm{F}>0$ and $\mathrm{S}$

Between $\mathrm{Cl}$ and $\mathrm{F}, \mathrm{Cl}$ has higher electron gain enthalpy then the $F,$ since the incoming electron experiences a greater force of repulsion because of small size of $F-$atom. Similar is true in case of $O$ and $S$, i.e. the electron gain enthalpy of $S$ is higher as compared to $O$ due to its small size. Thus, the correct order of electron gain enthalpy of given elements is

$\mathrm{O}<\mathrm{S}<\mathrm{F}<\mathrm{Cl}$