The correct answer is option 4: the Law of combining volumes.

This law states that when two or more gases react with each other, their volumes will bear a simple ratio. This means that the volumes of the reactant gases and the product gases will have a proportional relationship.

Option 1, the Law of mass action, is not related to the volumes of gases. It is actually a principle that describes the rate at which a chemical reaction proceeds, based on the concentration of the reactants.

Option 2, the Law of multiple proportions, is also not directly related to the volumes of gases. It states that when two elements combine to form more than one compound, the ratios of the masses of one element that combine with a fixed mass of the other element will be in small whole number ratios.

Option 3, the Law of reciprocal proportions, is not relevant to the question. This law deals with the ratios of the masses of elements in two different compounds that react with the same mass of a different element.

Therefore, the correct answer is option 4, the Law of combining volumes, which describes the relationship between the volumes of gases in a chemical reaction.