In which law are mass relations among three elements studied?

Correct answer: Law of reciprocal proportions

The direct answer is D: the law of reciprocal proportions studies mass relations among three elements. To understand it from the beginning, imagine three elements A, B and C. A and B may separately combine with C, and A and B may also combine with each other. The law compares the relevant combining masses. This is different from laws that describe only one compound or total mass. Option D is correct because it names the law specifically connected with three-element mass relationships. Option A, conservation of mass, says that matter is neither created nor destroyed in a chemical reaction; the total mass of reactants equals the total mass of products. It does not specifically define the three-element comparison here. Option B, definite proportions, says that a pure compound always contains the same elements in the same fixed mass ratio, such as water having a fixed composition. It usually concerns one pure compound. Option C, multiple proportions, says that when two elements form more than one compound, the masses of one element combining with a fixed mass of the other are in simple whole-number ratios. It concerns two elements, not the three-element reciprocal comparison. Therefore D is the precise answer. Memory cue: definite = fixed one compound; multiple = simple ratios in different compounds; reciprocal = three elements.