Answer : 3. "Joseph Thomson"
Explanation :

Sir J.J. Thomson, a British physicist, is credited with the discovery of the electron. In 1897, Thomson conducted experiments using cathode ray tubes and observed that cathode rays (streams of charged particles) were negatively charged. Through these experiments, he deduced the presence of a negatively charged particle, later named the electron.

Thomson's discovery of the electron was a groundbreaking contribution to the understanding of atomic structure and laid the groundwork for later developments in atomic and particle physics. The discovery of the electron marked a crucial step in the exploration of the subatomic world.