Who discovered that hydrogen on burning produces water? 

Henry Cavendish 

The correct answer is option 2. Henry Cavendish.

Henry Cavendish, an English scientist, is credited with the discovery that hydrogen, when burned, produces water. Cavendish conducted groundbreaking experiments on the properties of hydrogen gas in the late 18th century.

In one of his experiments, Cavendish reacted hydrogen gas (\(H_2\)) with oxygen gas (\(O_2\)) in controlled conditions. He ignited the mixture of hydrogen and oxygen, and observed the resulting reaction. Cavendish observed that when hydrogen gas burns in the presence of oxygen, water vapor (\(H_2O\)) is produced as a product.

The chemical reaction that occurs when hydrogen burns in oxygen can be represented by the equation:

\[ 2H_2(g) + O_2(g) \rightarrow 2H_2O(g) \]

This reaction demonstrates the combustion of hydrogen, where hydrogen molecules (\(H_2\)) react with oxygen molecules (\(O_2\)) to form water molecules (\(H_2O\)). This discovery was significant as it provided evidence for the composition of water and the role of hydrogen in its formation.

Cavendish's experiments paved the way for further research into the nature of chemical reactions and the understanding of the composition of substances. His discovery that hydrogen burns to produce water contributed to the foundation of modern chemistry and our understanding of chemical reactions.