Electron affinity is the amount of energy released when an electron comes in a vicinity of an isolated gaseous atom.
In case of O- and F- due to negative field, the incoming electron will face repulsion due to which energy is absorbed and will not be released.
In case of F2, the electron must be attracted strongly towards nucleus but because of small in size the electron will be repelled by valence electrons. Hence energy released will be less.
Now in case of Cl2 the repulsion factor is not there as much as in F2, due to large size of Cl.
Therefore electron affinity of Cl is highest.
You have rated this answer 6/10