The correct order of electron affinities of N, O, S and cl is :

Generally, electron affinity increases as we move from left to right in a period and decreases as we go down in a group. But electron affinities of second period elements (such as N, O) are less negative as compared to corresponding third period element. This is because of the small atomic size of second period elements.Hence, the correct order of electron affinities is N < O < S < Cl.