The process that is NOT endothermic in nature is :-

$\mathrm{Ar}_{(g)}+\mathrm{e}^{-} \rightarrow \mathrm{Ar}_{(\xi)}^{-}$

$\mathrm{H}_{(\mathrm{g})}+\mathrm{e}^{-} \rightarrow \mathrm{H}_{(\mathrm{g})}^{-}$

$\mathrm{Na}_{(z)} \rightarrow \mathrm{Na}_{(z)}^{+}+\mathrm{e}$

$\mathrm{O}_{(\mathrm{g})}^{-}+\mathrm{e}^{-} \rightarrow \mathrm{O}_{(\mathrm{g})}^{2-}$