Which element has the highest electron gain enthalpy?

Which of the following represents the correct order of increasing electron gain enthalpy with negative sign for the elements $O$,$S$,$F$ and $Cl$?

The correct order of electron affinity of $B, C, N, O$ is

The formation of the oxide ion,$O^{2-}_{(g)}$,from oxygen atom requires first an exothermic and then an endothermic step as shown below:
$O_{(g)} + e^- \to O^{-}_{(g)} ; \Delta_f H^{\Theta} = -141 \ kJ \ mol^{-1}$
$O^{-}_{(g)} + e^- \to O^{2-}_{(g)} ; \Delta_f H^{\Theta} = +780 \ kJ \ mol^{-1}$
Thus,the process of formation of $O^{2-}$ in gas phase is unfavourable even though $O^{2-}$ is isoelectronic with neon. It is due to the fact that,

Which halogen has the highest value of negative electron gain enthalpy?

Which elements show an anomalous trend in the negative value of electron gain enthalpy within their respective groups?