The correct answer is option 2: Law of definite proportion. The Law of definite proportion states that a given chemical compound always contains its elements in the same proportion by mass. In other words, the elements in a compound are combined in a fixed ratio.

In this case, calcium carbonate, whether found naturally as limestone or synthesized from quick lime, has the same composition, meaning it always has the same ratio of calcium, carbon, and oxygen atoms. This confirms the Law of definite proportion because the elements are combined in a fixed ratio regardless of whether the calcium carbonate is obtained naturally or produced synthetically.

Option 1, the Law of conservation of mass, refers to the principle that mass is neither created nor destroyed in a chemical reaction. While this law is generally applicable, it is not specifically demonstrated by the observation of calcium carbonate compositions.

Option 3, the Law of multiple proportion, states that when two elements combine to form different compounds, the ratio of masses of one element in the compounds will be a simple multiple of the ratio of the other element. This law is not applicable here as calcium carbonate is a single compound.

Option 4, Avogadro`s law, relates the volume of a gas to the number of gas particles present, and is not