Answer: Elements combine in a fixed ratio by their mass.
Step-by-step explanation:
Law of definite proportion states that the elements combining to form compounds always combine in a fixed ratio by their mass.
For Example: Taking a compound known as water
Here, 2 grams of hydrogen combines with 16 grams of oxygen.
So, in every molecule of water, hydrogen and oxygen will combine in a fixed ratio of 1 : 8 by their mass.
Hence, elements combine in a fixed ratio by their mass.