What is the nucleus of an atom primarily composed of?

The nucleus of an atom is primarily composed of protons and neutrons. Protons are positively charged particles, while neutrons have no charge. Together, they make up the majority of the atom's mass, as electrons, which are much lighter, orbit around the nucleus but do not contribute significantly to the atom's overall mass.

The presence of both protons and neutrons in the nucleus is essential for the stability of the atom; protons determine the element's identity and its positive charge, while neutrons contribute to the mass and can influence nuclear stability. The combination of these two types of nucleons (protons and neutrons) is what allows atoms to exist in various isotopes, which are atoms of the same element with different numbers of neutrons.

In contrast, the choices that involve electrons or only protons do not accurately reflect the composition of the nucleus, as electrons are not found in the nucleus but rather in electron clouds outside of it. Thus, the composition of the nucleus as protons and neutrons is critical for understanding atomic structure and stability.