Answer is: d. Mg3N2.
Mg₃N₂ is magnesium nitride.
Nitrogen (nonmetal) in this chemical compound has oxidation number -3 and magnesium (metal) has oxidation number +2.
Smallest common multiple for 3 and 2 is 6.
number of nitrogen atoms = 6 ÷ 3.
number of nitrogen atoms = 2, there are two nitrogen atoms.
number of magnesium atoms: 6 ÷ 2.
number of magnesium atoms = 3.