Samples of water have been sourced from a river, a tap and a rainwater harvesting tank. However, upon testing, all the samples contain hydrogen and oxygen in the ratio of 1 : 8 by mass irrespective of the source of water. Which of the following laws does this prove?

This is correct. In a pure chemical compound, elements are always present in a definite proportion by mass. This is known as the law of definite proportions. In this case, if we decompose 100 grams of pure water by passing electricity through it, then we get 11 grams of hydrogen and 89 grams of oxygen, showing that hydrogen and oxygen are combined together in water in the same constant proportion of 1 : 8 by weight.