The correct answer is: C. the number of nucleons.
A nucleon is a subatomic particle that is found in the nucleus of an atom. There are two types of nucleons: protons and neutrons. The number of protons in the nucleus of an atom is called the atomic number, and it determines the chemical element of the atom. The number of neutrons in the nucleus of an atom is called the neutron number, and it can vary for atoms of the same element. The mass number of an atom is the sum of the atomic number and the neutron number.
Option A is incorrect because the number of protons is equal to the atomic number, not the mass number.
Option B is incorrect because the number of protons and electrons is equal to the atomic number, not the mass number.
Option D is incorrect because the number of neutrons is not equal to the mass number. The mass number is equal to the sum of the atomic number and the neutron number.