Answer: B. number of protons
The atomic number is the number of protons present in the nucleus of an element. The atomic number is often regarded as the proton number and it determines the chemical properties of the element. In a chemical neutral atom or in an uncharged atom, the number of protons equals the number of electrons and as such, the proton number can be the electron number. The sum of the atomic number (number of protons) and the number of neutrons in an atom gives the mass number of that atom.