Final answer:
The Law of Definite Proportions indicates that a chemical compound consistently contains elements in fixed proportions by mass, exemplified by water's consistent composition of hydrogen and oxygen.
Step-by-step explanation:
The Law of Constant Composition, also known as the Law of Definite Proportions, states that a given compound will always have the same ratio of elements by mass. This means that no matter the source or amount of a chemical compound, the elements that make it up are present in constant ratios by mass. For instance, water (H2O) will always consist of 11.19% hydrogen and 88.81% oxygen by mass, regardless of its origin or the method of preparation.