Final answer:
The smallest particle among protons, neutrons, electrons, and positrons is the electron, which is significantly less massive than the other particles.
Step-by-step explanation:
The smallest of the particles listed—a proton, neutron, electron, and positron—is the electron. An electron is a lepton, specifically a fundamental particle with a very small mass. Comparatively, protons and neutrons are nucleons with nearly the same mass, and both are much more massive than electrons. For instance, a proton's mass is approximately 1,836 times that of an electron, and a neutron's mass is about 1,839 times the mass of an electron. A positron, on the other hand, has the same mass as an electron but differs by having a positive electric charge. When considering size in terms of mass, the electron is indeed the smallest since both the proton and the positron have significantly greater mass.