Which observation supports the law of constant proportions for water?

The correct option is Hydrogen and oxygen ratio is fixed.

The Law of Constant Proportions (also known as the Law of Definite Proportions) states that a chemical compound always contains its component elements in a fixed ratio by mass, regardless of its source or method of preparation.

For water ($H_{2}O$), this means that hydrogen and oxygen are always chemically combined in a fixed ratio. Specifically, the mass ratio of hydrogen to oxygen in any sample of pure water is always approximately 1:8 (or 2:16), which means that the ratio of hydrogen and oxygen is fixed. Therefore, this observation directly supports the law of constant proportions.