The correct order of electron gain enthalpy is :

Oxygen is the second most electronegative element in comparison to fluorine. In group - 16 family $(\mathrm{O}, \mathrm{S}, \mathrm{Se}, \mathrm{Te})$, O-atom is smallest in size. So, electron density on O-atom is very high in group -16 During addition of a free electron to gaseous $\mathrm{O}$-atom,<br/><br/> $\mathrm{O}(\mathrm{g})+\mathrm{e}^{-} \longrightarrow \mathrm{O}^{-}(\mathrm{g})$<br/><br/> We have to supply a significant amount of energy (endothermic) to overcome the electrostatic repulsion between the approaching electron and O-atom of very high electron density. So, the net value of electron affinity (EA) or (negative) electron gain enthalpy $\left[\Delta_{\mathrm{eg}} H\right.$ or $\left.\left|\Delta_{\mathrm{eg}} H\right|\right]$ of oxygen decreases to a higher extent in comparison to other elements of group -16 who have larger size and lower electronegativity. <br/><br/> So, the correct order of EA or $\left|\Delta_{\mathrm{eg}} \mathrm{H}\right|$ of group $-16$ elements will be $\mathrm{S}$ $>\mathrm{Se}>\mathrm{Te}>\mathrm{O}$