Smaller sub-particles of the proton and neutron are called ___.

• A. Electrons
• B. Atoms
• C. Protons
• D. Quarks

Answer: (D)

The correct answer is quarks because they are the fundamental constituents of protons and neutrons, which are collectively known as nucleons. Protons and neutrons are not indivisible; they are made up of quarks, which are held together by the strong force mediated by particles called gluons. Specifically, a proton is made up of three quarks (two up quarks and one down quark), while a neutron consists of one up quark and two down quarks.

In contrast, electrons are elementary particles that exist outside the nucleus of an atom and do not constitute protons or neutrons. Atoms themselves are made up of protons, neutrons, and electrons but are not sub-particles of protons and neutrons. Lastly, protons are one type of nucleon and cannot refer to the smaller parts within them, which are the quarks. Therefore, the term "quarks" accurately describes the smaller sub-particles that make up protons and neutrons.