What does the law of definite proportion assert?

The law of definite proportion asserts that pure compounds are composed of the same elements in the same proportion by mass, regardless of the source of the compound. This means that if you have a specific chemical compound, such as water (H2O), it will always contain hydrogen and oxygen in the exact ratio of 2:1 by mass. This principle is fundamental in chemistry because it reinforces the concept that the composition of a given substance is consistent, allowing chemists to predict the behavior and properties of compounds based on their chemical makeup.

Other options, while related to different concepts in chemistry, do not accurately convey the meaning of the law of definite proportion. For instance, a compound cannot consist of only one element; that would be an element rather than a compound. The atomic mass of an element being constant refers to a separate concept regarding atomic theory, and the assertion that compounds can vary in composition contradicts the law of definite proportions, as it implies inconsistency in the elemental makeup of compounds.