An element that is oxidized is one that loses electrons, losing electrons increases its oxidation state. Now, the more electrons there are in the last energy level of the element, that is, valence electrons, the easier it will be for this element to lose them. We can identify valence electrons in the periodic table, the group to which the element belongs indicates how many valence electrons it has. Let's see what the oxidation state of the elements is:
F:7
Li:1
N:5
Be:2
O:6
We see that Lithium is the one with the fewest valence electrons, it has only one valence electron, so this will be the element that is most likely to be oxidized.
