What type of particle is responsible for making up most of the mass of an atom?

The correct choice indicates that protons primarily account for the mass of an atom. Within the atomic structure, both protons and neutrons, which are collectively referred to as nucleons, reside in the nucleus and contribute significantly to the overall mass, with protons being a key component.

Protons have a mass that is approximately 1 atomic mass unit (amu), while neutrons also have a mass close to 1 amu. Electrons, in contrast, have a much smaller mass—around 1/1836 of a proton's mass—making them negligible when calculating the total mass of an atom. The electron cloud refers to the region where electrons are likely to be found around the nucleus, but it does not contribute to the mass in a significant way due to the minimal mass of the electrons compared to the protons and neutrons. Thus, the overwhelming majority of an atom's mass comes from the protons, affirming the correctness of the choice that identifies them as a crucial component contributing to atomic mass.