X(s)→X(g), requires energy to change the state from solid to gas. O−(g) has already one electron more than protons in nucleus. Therefore, it will repel the next incoming electron. So, energy is required to make O−(g)⟶O2−(g) O(g)⟶O+(g)+e− is an endothermic process. It requires energy to remove the electron from O(g). Only option (d) indicates release of energy due to electron gain enthalpy [ ∆H=(−) ve ].