Which one of the following arrangements represents the correct order of least negative to most negative electron gain enthalpy for $C, Ca, Al, F$ and $O$?

Which of the following processes is endothermic?

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

Which of the following elements will have the lowest electron affinity?

The first electron affinity of $C, N$ and $O$ will be of the order

The electron gain enthalpy (with negative sign) of fluorine is less than that of chlorine due to: