The correct answer is Neutron.

James Chadwick is best known for his discovery of the neutron in 1932. Prior to this discovery, the atom was understood to consist of protons and electrons. However, this model couldn't fully explain the observed masses and behavior of atomic nuclei. Chadwick's experiments, involving bombarding beryllium with alpha particles, revealed the existence of a neutral particle with a mass approximately equal to a proton. This particle, lacking an electric charge, was named the neutron. This discovery revolutionized our understanding of atomic structure and nuclear physics, opening the door to further advancements such as nuclear fission and the development of nuclear weapons.

Options 1, 2, and 4 are incorrect. The wave theory of light was primarily developed by figures such as Huygens and later refined by Fresnel and others. Thermal ionization is a physical process, not a specific discovery attributed to a single person. The electron was discovered earlier by J.J. Thomson.