In which of the following processes is the least energy required?

In which of the following options are the elements correctly arranged with respect to their negative electron gain enthalpies?

The process requiring absorption of energy is

The least stable ion is

The formation of the oxide ion $O_{(g)}^{2-}$ requires first an exothermic and then an endothermic step as shown below. This is because
$O_{(g)} + e^{-} \rightarrow O_{(g)}^{-}; \Delta H^{o} = -142 \ kJ \ mol^{-1}$
$O_{(g)}^{-} + e^{-} \rightarrow O_{(g)}^{2-}; \Delta H^{o} = 844 \ kJ \ mol^{-1}$

Select the equations that represent an endothermic step.