b
\((b)\) If we consider the opposite process :
\((a)\, F_{(g)} +e^- \to F^-_{(g)}\,\,\,\,\,\,\,\,\, ; \,\,\,\,\,\,\,\, \Delta H_1\)

\((b)\, P_{(g)} +e^- \to P^-_{(g)}\,\,\,\,\,\,\,\,\, ; \,\,\,\,\,\,\,\, \Delta H_2\) 

\((c)\, S_{(g)} +e^- \to S^-_{(g)}\,\,\,\,\,\,\,\,\, ; \,\,\,\,\,\,\,\, \Delta H_3\) 

\((d)\, Cl_{(g)} +e^- \to Cl^-_{(g)}\,\,\,\,\,\,\,\,\, ; \,\,\,\,\,\,\,\, \Delta H_4\) 

Order of energy releases Is : \(\Delta H_4 > \Delta H_1 > \Delta H_3 > \Delta H_2\)

So,  \(P^-_{(g)} \to P_{(g)} + e^- ;\)  Requires least energy.