The correct answer is (D) Quarks because they are the fundamental subatomic elementary particles that physically compose the protons and neutrons inside an atom's core.
Subatomic Substructure: While the nucleus (core) of an atom is composed of protons and neutrons, these are not elementary blocks. Modern particle physics proves they are constructed from smaller point-like units known as quarks.
Quark Composition: Every proton is built from two 'up' quarks and one 'down' quark, while every neutron is built from one 'up' quark and two 'down' quarks, held together by gluons via the strong nuclear force.
Incorrect Options:
(A) protons and the nucleons is incorrect because 'nucleon' is simply the collective umbrella term for any particle in the nucleus, making the phrase grammatically and logically redundant.
(B) Neutrons is incorrect because neutrons represent only half of the larger composite structures in the nucleus and are themselves made up of smaller constituent quarks.
(C) Electrons is incorrect because electrons are light lepton particles that orbit outside in energy shells rather than forming the compact atomic core.