Which statement best explains how isotopes can have different masses and still be the same element?

a.
the quantity of protons identifies the element but changes if the mass of the isotope differs from what is shown on the periodic table.

b.
the sum of the protons and electrons equals the mass of the isotope, and the neutron amount is equal to the atomic number.

c.
the sum of the electrons and neutrons equals the isotope's mass with the proton amount being given by the atomic number.

d.
the quantity of protons identifies the element and cannot change; therefore, the neutron amount changes, ultimately changing the overall mass of the atom.