The correct answer is option A.
Atoms of isotopes can be distinguished from atoms of different elements because isotopes always have the same number of protons as all other atoms of that same element.
Isotopes of different atoms of the same element have the same number of protons and electrons, but different numbers of neutrons.
Different isotopes of the same element have the same atomic number (as the number of protons are same) but different atomic masses (as the number of neutrons are different and atomic mass is the summation of the number of protons and the number of neutrons).
