electrons are arranged in atoms in increasing energy levels.
Valence energy level is the outermost energy level
the electronic configuration for each of these elements and the valence electrons are given below.
S - 2,8,6 valence electrons are 6
N - 2,5 - valence electrons are 5
Al - 2,8,3 - valence electrons are 3
C - 2,4 - valence electrons are 4
to fill an energy shell there should be 8 valence electrons. So to gain 3 electrons to make it a total of 8 valence electrons;
the number of valence electrons that should already be in the element is 8 - 3 = 5
element with 5 valence electrons is N
answer is B. N