According to the Law of Definite Composition, a compound is composed of what?

The Law of Definite Composition states that a given compound will always be composed of its constituent elements in fixed, definite proportions by mass, regardless of the sample size or the source of the compound. This principle reflects that every sample of a compound, such as water or carbon dioxide, contains the same ratio of elements. For example, water consistently consists of two hydrogen atoms for every one oxygen atom (H2O), which means that by mass, water is composed of approximately 11.2% hydrogen and 88.8% oxygen, regardless of where the water is sourced from or how much of it there is.

This law contrasts with variations in other types of substances, such as mixtures, where the composition can indeed differ based on the method of preparation or combination of components. Thus, the fixed ratio as expressed in the correct choice illustrates the unchanging nature of compound composition according to the law.