The correct answer is d protons plus neutrons.
The mass number of an atom is a measure of the total number of protons and neutrons in its nucleus. This number is important because it helps define the atom’s identity and stability. Protons, which are positively charged, contribute to the mass, and neutrons, which are neutral, also add to the mass of the atom. Electrons, while they have a charge, are much less massive compared to protons and neutrons. Therefore, when discussing mass number, we consider only protons and neutrons.