Which of the following is an example of the Law of Constant Proportions?

The Law of Constant Proportions, which states that a chemical compound always contains the same elements in the same proportion by mass, regardless of the source or method of preparation of the compound. 

In water (${\mathrm{H}}_2\mathrm{O}$), the ratio of hydrogen to oxygen is always 2:1 by mass, demonstrating this law.