Answer: a . is determined by the number of protons and neutrons it contains.
Explanation:
Atomic number is defined as the number of protons or the number of electrons that are present in an electrically neutral atom.
Atomic number = Number of protons = number of electrons
Mass number is defined as the sum of number of protons and neutrons that are present in an atom.
Mass number = Number of protons + Number of neutrons
As the mass of electron is negligible as compared to mass of neutron and proton, the number of electrons does not influence the atomic mass of an element.