Final answer:
The components that most significantly contribute to the mass of an atom are the proton and neutron. These two subatomic particles have a mass of roughly 1 atomic mass unit each and are located in the atom's nucleus, whereas the mass of an electron is too small to significantly influence the atom's overall mass.
Step-by-step explanation:
The combination that makes up most of the mass of an atom is c) Proton and Neutron. Atoms are composed of three types of subatomic particles: protons, neutrons, and electrons. Because both protons and neutrons have a mass of approximately 1 atomic mass unit (amu), they contribute significantly to the atomic mass of an atom. On the other hand, an electron has a very small mass, around 1/2000th of the mass of a proton or neutron, making its contribution to the overall mass of an atom negligible.
The mass number (A) of an atom is the total number of protons and neutrons in its nucleus and defines the atom's mass. Since electrons have such a minuscule mass, they do not significantly affect the overall mass, and thus the mass number only includes protons and neutrons. The atomic mass of a single atom is approximately equal to its mass number, which is essentially the sum of protons and neutrons in the nucleus.