Question

states that every sample of the same substance contains identical elements in the same fixed ratio by mass.

Explanation:

The law of definite proportions, also known as Proust's law, states that a given chemical compound always contains its component elements in fixed ratio (by mass) and does not depend on its source and method of preparation. Avogadro's law states that equal volumes of gases, at the same temperature and pressure, have the same number of molecules. The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. Boyle's law states that the pressure of a given mass of gas varies inversely with its volume at constant temperature.

Answer:

The law of definite proportions