The correct answer is option 1: James Chadwick.

Neutrons are subatomic particles found in the nucleus of an atom, along with protons. They have no electrical charge, making them neutral.

James Chadwick discovered the existence of neutrons in 1932 through a series of experiments. He built upon the work of Ernest Rutherford, who had proposed the concept of a nucleus containing positively charged protons.

Chadwick`s experiments involved bombarding beryllium atoms with alpha particles, which resulted in the emission of a radiation that was not affected by electric or magnetic fields. He realized that this radiation consisted of particles with a similar mass to protons but with no electrical charge. These particles were later named neutrons.

It`s important to note that Ernest Rutherford, mentioned in option 2, made significant contributions to the understanding of atomic structure and the existence of the nucleus but did not discover neutrons. J.J. Thomson, mentioned in option 3, is known for the discovery of the electron, not neutrons. John Dalton, mentioned in option 4, proposed the atomic theory but did not discover neutrons either.