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 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 contain equal numbers 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 pressure of a given mass of an ideal gas is inversely proportional to its volume at a constant temperature.

Answer:

The law of definite proportions