Which of the following is defined as the mass of an atom based on the mass of an atom of carbon-12?

atomic charge

atomic mass

atomic number

isotope

This would be the atomic mass. In an atom of carbon-12, there are 6 protons and 6 neutrons at rest (electrons have a negligible mass and are usually not part of the overall mass calculation). All atomic masses are based off the measurements of this specific iteration of carbon.