Which law accurately reflects the proportions of elements in a chemical compound?

The Law of Constant Composition is the correct choice because it states that a given chemical compound always contains its component elements in fixed, definite proportions by mass. For example, water (H₂O) is composed of hydrogen and oxygen in a ratio of about 1:8 by mass, regardless of the source of the water. This principle is fundamental in chemistry, as it ensures that any sample of a pure compound will always have the same elemental ratio.

In contrast, the Law of Conservation of Mass relates to the total mass of reactants being equal to the total mass of products in a chemical reaction, but it does not specifically address proportions within a compound itself. The Law of Multiple Proportions describes how elements can combine in different ways to form different compounds, leading to various mass ratios, but it is not about the fixed composition of a single compound. Lastly, the Law of Gaseous Diffusion pertains to the behavior of gases and their rates of diffusion, which is unrelated to the proportions of elements in compounds.