The process that is NOT endothermic in nature is:

🧠 Hydrogen completing its $1s^2$ shell by gaining an electron is exothermic — the only non-endothermic process here Endothermic processes need energy input. Ionization is always endothermic. Adding electrons to noble gases is endothermic (they don't want more). Adding electrons to anions is endothermic (repulsion). Adding electrons to neutral atoms with unfilled shells is usually exothermic.

🗺️ Check each process (a) $\ce{Ar(g) + e^- -> Ar^-(g)}$: Ar has filled $3p^6$. The new electron must go into 4s — strongly unfavorable. Endothermic (~$+96\,kJ/mol$). (b) $\ce{H(g) + e^- -> H^-(g)}$: H has $1s^1$. Gaining one electron completes the $1s^2$ shell — a stable configuration. Exothermic ($\Delta H = -73\,kJ/mol$). NOT endothermic. (c) $\ce{O^-(g) + e^- -> O^{2-}(g)}$: pushing an electron into an already negative ion. Strong repulsion. Endothermic ($+780\,kJ/mol$). (d) $\ce{Na(g) -> Na^+(g) + e^-}$: ionization. Always endothermic ($+496\,kJ/mol$).

Only (b) is exothermic.

⚠️ The trap Option (d) Na ionization tempts students who confuse ionization (always endothermic — must SUPPLY energy to remove an electron) with electron gain (often exothermic). The arrow direction matters: $\ce{Na -> Na^+ + e^-}$ is electron LOSS, which always requires energy. Read the arrow before classifying.

$\boxed{\text{Answer: (b)}}$