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

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

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

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.