(i) The electron gain enthalpy of O is positive while that of S is negative because of repulsions between the electrons on O- and the additional incoming electron.
(ii) Electron gain enthalpy of O is more positive than that of S- due to small size which causes repulsions between O- and the incoming electron is much more than in S-
(iii) The electron gain enthalpy of N is positive while that of P is negative because of repulsion between N- and the incoming electron.