Question

Four samples of water are collected from different sources. Each sample on analysis was found to contain same percentage of oxygen.
Which law of chemical combination is demonstrated by the above observations?

Answer 1

Law of Definite Proportions =>

Law of Definite Proportions states that in a chemical substance , the elements are present in definite proportions by mass.

In the four samples of water , the percentage of oxygen is definite or is the same.

Answer 2

law of definite proportions:- 

it states that a pure chemical compound always contains same elements combined together in the same proportion by mass.

they have same percentage of oxygen, that's why 'law of definite proportions' is demonstrated by the observation.