The correct option is B magnesium nitride, combination reaction
3Mg+N2→Mg3N2
As do many active metals, Magnesium reacts with dinitrogen gas to form magnesium nitride.
Alkaline earth metals react with nitrogen to give respective metal nitrides whereas alkali metals do not react except Li.
Combination reaction is a reaction in which reactants combine to form a single product. In this reaction, only magnesium nitride is formed and thus it is a combination reaction.