We can first rule out choices B and D because in decomposition reactions, one reactant is decomposed or broken up to form two or more products. Choices B and D are incorrect because they both have two reactants.
Then we need to determine which choice, A or C, is correctly balanced. Choice A is incorrectly balanced because there are 2 moles of magnesium on the reactants side but only one mole of magnesium on the products side. Therefore, C is the correct choice because it is correctly balanced.
Hope this helps.
