Which of the following ions is most unlikely to exist?

Which of the following statements is correct?

The correct order of electron gain enthalpy of $N$,$O$,$Cl$,$Al$ is

Electron affinity is the lowest for

What is the value of electron gain enthalpy of $Na^{+}$ if $IE_{1}$ of $Na$ is $5.1 \ eV$? .......... $eV$

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,