What is the correct order of chemical reactivity in terms of oxidising property for the following elements: F, Cl, O, and N
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.