Which particles account for most of an atom's mass?

The option indicating that protons and neutrons account for most of an atom's mass is correct because both of these particles are located in the nucleus of the atom, where the majority of the atom's mass is concentrated. Protons have a mass of approximately 1 atomic mass unit (amu) and carry a positive charge, while neutrons also have a mass around 1 amu but carry no charge.

Electrons, on the other hand, are much lighter than protons and neutrons, with a mass that is roughly 1/1836 of a proton's mass. While electrons do play a crucial role in chemical bonding and the overall charge of an atom, their contribution to the total mass of the atom is negligible compared to that of protons and neutrons.

Positrons are the antiparticles of electrons and also do not contribute significantly to the atom's mass when present, as they are involved primarily in particle physics rather than in the structure of ordinary matter. Thus, the combined mass of protons and neutrons makes them the primary contributors to the overall mass of an atom.