Which of the following is NOT one of the three main parts of an atom?

The reason "D" is the correct answer is that quarks are not considered one of the three main parts of an atom. The three fundamental components of an atom are electrons, neutrons, and protons. Electrons are negatively charged particles that orbit the nucleus, while neutrons and protons make up the nucleus itself. Neutrons have no charge, and protons are positively charged.

Quarks, on the other hand, are elementary particles that combine to form protons and neutrons. They are not standalone components of an atom but rather the constituents that make up those nucleons. Thus, in the context of this question, recognizing that electrons, neutrons, and protons are the primary parts of an atom provides clarity on why quarks do not belong in this category.