The equation used to represent the electron gain enthalpy is

• A
  $X_{(g)} + e^{-} \longrightarrow X^{-}_{(g)}$
• B
  $X_{(s)} + e^{-} \longrightarrow X^{-}_{(g)}$
• C
  $X_{(g)} \longrightarrow X^{+}_{(g)} + e^{-}$
• D
  $X_{(s)} \longrightarrow X^{+}_{(g)} + e^{-}$