Answer:
b. number of protons
Explanation:
The definition of the atomic number is the number of protons.
It is possible for two atoms to have the same mass number but a different number of protons (for example one atom with 23 protons and 25 neutrons, and one atom with 24 protons and 24 neutrons). This is because the mass number is equal to the sum of the number of protons and neutrons.
