Select the best oxidising agent from the given options
Let's see the electronic configuration of these elements.
Li - 2, 1.
N - 2, 5.
O - 2, 6.
F - 2, 7.
From the electronic configuration, it is clear that fluorine requires only one electron to obtain a stable octet configuration. Whereas oxygen requires two electrons and nitrogen requires three electrons. Lithium needs to donate one electron in order to obtain a stable duplet(The first orbit can only have 2 electrons). The best oxidising agent is one which can easily reduce itself. According to the definition of reduction, an element is said to be reduced if it gains an electron or its oxidation number value decreases. Since fluorine requires only one electron it can easily reduce itself compared to the other options. Therefore it is the best oxidising agent in the given options.
Note: Since lithium needs to donate an electron it is a better reducing agent than all the other options.