This is a textbook question. I am confused as to which factors i should consider while sorting out these elements. If I take electron gain enthalpy, then Cl should be greater than F, but if i consider electronegativity F is greater than Cl . I'd be grateful to anyone who provides some idea..
See, F is the strongest oxidising agent in the entire periodic table. Also, according to periodic table trends, oxidising character decreases down the group and increases across a period. You should check the reactivity of any element according to the question given, here, according to the oxidising character. If nothing is given, usually IE and Electronegativity is considered. Therefore, I think the order should be F>O>Cl>N. Cl>N because Cl is more oxidising than N. I must add that a simple Google search would have yielded the same result.