An atom of carbon-12 and an atom of carbon-14 differ in (1) atomic number (2) mass number (3) nuclear charge (4) number of electrons

(2) mass number

the number after the two carbons is the mass number, the two carbons are different isotopes of each other

isotopes have the same number of protons but different number of neutrons, neutrons and protons make up the atomic mass number, so if you add neutrons to the atom the mass number increases

i hope that makes sense!