If a question mentions total reactant mass and total product mass which law is most likely?

Correct answer: Conservation of mass

The direct answer is option A: the law of conservation of mass. This law says that in a closed system the total mass of substances before a chemical reaction equals the total mass after it. Step 1: notice the key words “total reactant mass” and “total product mass.” Step 2: recognize that these compare mass before and after a reaction. Step 3: apply conservation of mass: total reactant mass = total product mass. Option A exactly names this law. Option B, definite proportions, says that a pure compound always contains its elements in a fixed mass ratio; it does not mainly compare total reactant and product masses. Option C, multiple proportions, compares different compounds made from the same elements. Option D, reciprocal proportions, concerns relationships among combining masses of elements in different compounds. Thus A is the correct clue-based answer. Memory cue: total mass before and after means conservation of mass.