The order of first electron affinity of O , S and Se :-

(A) O>S>Se (B) S>Se>O (C) Se>O>S (D) S>O>Se

The correct order for the electron affinities of the asked elements is as follows

                                                                      S > O > Se

We know the general trend of electron affinities that they reduce as we proceed down the group. However, the electron affinity of Sulphur is more than oxygen. This occurs due to the small size of oxygen atom. Thus the addition of an extra electron to the oxygen causes more inter-electronic repulsion, and hence results in lower value of electron affinity for oxygen. Remember electron affinity is the amount of energy released when an extra electron is added to a neutral gaseous atom, converting it into a negative ion. Negative electron affinity indicates that energy is given out when an atom accepts an electron. Hence more negative is the electron affinity, more is the energy released when an electron is added to an atom.