The correct statement about the law of conservation of mass is option-3: Matter can neither be created nor destroyed. This statement is a fundamental principle in chemistry, which states that the total mass of substances in a closed system remains constant over time. In other words, during a chemical reaction or a physical change, there is no net gain or loss of mass.

Option-1 is not correct because compounds can have varying proportions of elements, depending on the specific compound.

Option-2 is not directly related to the law of conservation of mass, but it is a separate law known as the law of multiple proportions. It states that when gases combine, the ratio of their volumes can often be expressed as small whole numbers. This principle helps determine the relative proportions of gases in a chemical reaction.

Option-4 is not directly related to the law of conservation of mass, but it is a statement known as Avogadro`s law. It states that equal volumes of gases, at the same temperature and pressure, contain an equal number of molecules. This concept is useful for understanding the behavior of gases.

In summary, only option-3 accurately describes the law of conservation of mass.