Concept:A balanced chemical equation ensures equal numbers of atoms of each element on both sides. This directly follows from the law of conservation of mass.Explanation:The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction.Therefore, the total mass of reactants must equal the total mass of products.To obey this law, the number of atoms of each element must be the same on the reactant and product sides.Thus, balancing a chemical equation is based on this law.Answer:D. Conservation of mass