The process that is $NOT$ endothermic in nature is :-

• A
  $Ar_{(g)} + e^{-} \rightarrow Ar_{(g)}^{-}$
• B
  $H_{(g)} + e^{-} \rightarrow H_{(g)}^{-}$
• C
  $Na_{(g)} \rightarrow Na_{(g)}^{+} + e^{-}$
• D
  $O_{(g)}^{-} + e^{-} \rightarrow O_{(g)}^{2-}$