Which French chemist summarised his experiment in 1806, and proved that the mass ratio of elements in a chemical compound is always the same, regardless of the source of the compound?

Joseph Proust

The correct answer is Option (1) → Joseph Proust

The French chemist who summarised his experiment in 1806, proving that the mass ratio of elements in a chemical compound is always the same, regardless of the source of the compound, was Joseph Proust.

His work on this concept is known as the Law of Definite Proportions or the Law of Constant Composition.