Oxidation number is basically the ability of an atom to loose or gain electrons in order to gain stability or in the influence of any other element. This strongly suggests that the electronic configuration influences whether the concerned atom can accept or donate electrons under appropriate conditions.
The valence electrons are directly related to oxidation number, thus more the valence electrons more is the possibility to donate electrons, and if it has a tendency to accept electrons then the number of empty orbitals increases the oxidation number.
For example, N has Z=7, with valence electronic configuration as 2s2 2p3, so it has tendency to either fulfill its external orbit by 3 electrons to complete its octate, or it can donate 3 electrons to gain positive charge an attain duplet thus gaining stability.
Thus from above option the maximum possible electrons that can be donated is 7 which is shown by option (iv).
*there is one more abstract possibility that option (ii) 3d3 4s2 shows tendency to accept 7 electrons to completely fill the d-orbital and one can even expect an oxidation state of -7, but due to instability of ion developed and low feasibility that kind of oxidation state is not exhibited.
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