The formation of the oixde ion, `O^(2+)(g)` from oxygen atom requires first an exothermic and then an endothermic step as shown below:
`O(g)+e^(-) rarr O^(-)(g), Delta H^(O)=-141kJ mol^(-)`
`O(g)+e^(-) rarr O^(-)(g), H^(O)=+kJ mol^(-)`
Thus the process of formation of `O^(2-)` in gas phase is neon. It is due to the fact that

Options

A. oxygen is more electronegativce
B. addition of electron in oxygen results in lrger size of the ion
C. electron repulsion outweights the stability gained by achieving noble gas configuration
D. `O^(-)` ion has comparatively smaller size than oxygen atom

Explanation

The process of formation of `O^(2-)` in gas phase is unfavourable even though `O^(2-)` is isoelectronic with neon because electron repulsion outweights the stability gaiped by achieving noble gas configuration.