Answer
D) the law of constant composition.
Step-by-step explanation
According to the law of constant composition or definite proportion, which states that a given chemical compound always contains its component elements in a fixed ratio (by mass) and does not depend on its source and method of preparation.
So a container of a solution of 5% HCl, all samples tested from this same bottle will have the same mass ratio of hydrogen to chlorine is an application of the law of constant composition.
The correct answer is option D) the law of constant composition.