Atoms' nuclei are primarily made up of which particles?

Atoms' nuclei are primarily composed of neutrons and protons. Protons carry a positive electrical charge, while neutrons are electrically neutral. Together, they make up the bulk of an atom's mass and define the identity of the element, as the number of protons determines the atomic number. Neutrons add stability to the nucleus by mitigating the repulsive forces between the positively charged protons.

Other particles mentioned in the choices, like electrons, are not found in the nucleus itself; they orbit around the nucleus and contribute to atomic structure but have a negligible mass compared to protons and neutrons. Positrons are the antimatter counterparts of electrons and do not play a role in the structure of atomic nuclei. Quarks and leptons are fundamental particles, but quarks are what compose protons and neutrons rather than being a description of the nucleus itself. Thus, the correct focus on neutrons and protons highlights the essential components that make up the nucleus of an atom.