With increase in size, tendency of nucleus to bound the outer electron decreases. Thus, an electron if added to an isolated gaseous element, the amount of energy released decreases with increase in size. However, the electron affinity of sulphur is higher than oxygen. This is because the incoming electron experience a greater force of repulsion due to smaller size of oxygen. Hence, to overcome this repulsion, some energy is utilized and less amount of energy is released. Thus, electron affinity is highest for S among the given.