the strongest oxidizing agent among the following is, oxygen, hydrogen, lithium, fluorine?

Answer 1

Oxidizing agents remove electrons from other atoms to complete a stable outer octet. Therefore, oxygen and fluorine are oxidzers. (Hydrogen and lithium, because they can lose electrons more easily than accept them, are reducing agents.)
So, of the two oxidizers which is better? It will be the one that is more electronegative. Electronegativity increases left to right on a row in the periodic table*. F is therefore more electronegative than O, so the winner is F.
'Chemistry is full of exceptions and this rule does not hold for the noble family (VIIIa) because their electron structures are fully satisfied.

Answer 2

Simply fluorine (even though oxygen is known as the "elctron hog.")

Answer 3

Oxygen!