How does the mass number of an atom relate to the number of neutrons and protons?

The mass number of an atom is fundamentally defined as the total count of nucleons, which includes both protons and neutrons in the nucleus. Each proton and neutron contributes approximately one atomic mass unit to the mass number. Therefore, if you know the number of protons (which determines the element) and the number of neutrons, you can add these two quantities together to arrive at the mass number.

For instance, if an atom has 6 protons (which identifies it as carbon) and 6 neutrons, the mass number would be 6 + 6 = 12. This relationship is essential for understanding the structure of an atom and how isotopes of an element can vary in the number of neutrons while maintaining the same number of protons. Thus, option C accurately captures this relationship by stating that the mass number is the sum of the number of protons and neutrons.