The correct order of electron affinity is: F>O>Cl

The correct order of electron affinity is Cl > F > O

On moving from left to right across a period, the electron affinity becomes more negative.
On moving from top to bottom in a group, the electron affinity becomes less negative.
Chlorine has more negative electron affinity than fluorine. Because adding an electron to fluorine 2p orbital causes greater repulsion than adding an electron to chlorine 3p orbital which is larger in size.
Note: The electron affinity (in kJ/mol) of Cl, F and O is and respectively.