Who gave the law of constant proportions? 

Proust 

The correct answer is option 3. Proust.

The Law of Constant Proportions, also known as the Law of Definite Proportions, was formulated by Joseph Louis Proust, a French chemist, in the late 18th century. This fundamental law of chemistry states that a given chemical compound always contains its constituent elements in fixed and definite proportions by mass, regardless of its source or method of preparation.

In other words, the relative masses of the elements in a compound are constant and do not change, regardless of the amount of the compound or how it is produced. For example, water (H2O) always consists of two hydrogen atoms and one oxygen atom in a fixed ratio by mass, with hydrogen and oxygen combining in a 1:8 mass ratio.

This law is illustrated by the example of water:

Regardless of whether water is obtained from a natural source, synthesized in a laboratory, or extracted from different sources, it always contains hydrogen and oxygen in the same fixed proportion by mass.

No matter how much water is analyzed, its composition remains constant, with hydrogen and oxygen combining in a mass ratio of approximately 1:8.

The Law of Constant Proportions played a crucial role in the development of modern chemistry by providing evidence for the existence of distinct chemical compounds with fixed compositions. It laid the foundation for the atomic theory proposed by John Dalton and contributed to the understanding of chemical reactions and stoichiometry.