Final answer:
The mass of an atom comes from the nucleus, where the mass is almost entirely attributable to the combined mass of its protons and neutrons.
Step-by-step explanation:
The mass of an atom comes predominantly from its nucleus, which is composed of protons and neutrons. Both protons and neutrons, collectively referred to as nucleons, have a mass of approximately 1 atomic mass unit (amu). In contrast to nucleons, electrons have a negligible mass and hence contribute very minimally to the overall mass of an atom. Given this, the mass of an atom is essentially determined by the sum of the masses of its protons and neutrons.
Each proton and each neutron has a mass of about 1 amu, with a neutron being just very slightly heavier. The contribution of protons and neutrons to the atom's mass can be expressed through the mass number (A), which equals the number of protons (Z) plus the number of neutrons (N), denoted by the formula A = N + Z. Therefore, the answer to the student's question is A. the nucleus.