The correct answer is Option 2: H. C. Urey.

Harold C. Urey, an American chemist, is credited with the discovery of heavy water. Urey made this important scientific breakthrough in 1931 while he was working at Columbia University in New York City.

Heavy water is a form of water in which the hydrogen atoms are replaced with deuterium, an isotope of hydrogen that contains both a proton and a neutron in its nucleus. This substitution results in a heavier molecular structure. Heavy water is denoted as D2O, where the "D" represents deuterium.

Urey`s discovery of heavy water was a significant contribution to the field of chemistry. He developed a technique known as the "exchange method" to separate heavy water from regular water. Urey and his team were able to achieve this by utilizing the differences in the boiling points of water and heavy water.

The discovery of heavy water had important implications in various scientific fields, particularly in nuclear physics and chemistry. Heavy water is commonly used as a moderator in nuclear reactors, as it can slow down the neutrons produced during fission reactions. It also played a crucial role in the development of the atomic bomb during World War II.

While the other options mentioned in the question are notable scientists in their respective fields, they are not associated with the discovery of heavy water. Heinrich Hertz was a German physicist known for his work on electromagnetic waves, but he did not discover heavy water. Gregor Mendel, an Austrian monk, is famous for his experiments in plant breeding and establishing the laws of inheritance, but he had no involvement in the discovery of heavy water. Joseph Priestley, an English scientist, is known for his discovery of oxygen, but he was not involved in the discovery of heavy water either.

Therefore, the correct answer is H. C. Urey, who made the discovery of heavy water and contributed significantly to our understanding of this unique form of water.