Answer
(b) the sum of its protons and neutrons
Work Step by Step
Essentially, the atomic mass is the sum of all of the subatomic particles (protons, neutrons, and electrons). However, the electrons weigh practically zero, so the atomic mass is just the sum of the protons and neutrons.
The answer is not a) because the number of protons in the atom is defined as the atomic number.
The answer is not c) because electrons are not accounted for in the atomic mass, just the protons and neutrons.
The answer is not d) because the average of all of the mass numbers of all of its isotopes is defined as the atomic weight.
