The correct answer is B. Isotopes are atoms having the same number of protons of the same element but with a different number of neutrons. Base on the choices, letter B is the only option that follows the description of an isotope. Choice A and choice D have different elements so we can cross them out Choice C has the same elements but there is no difference in mass.
Therefore, choice B is the isotope. It has the same element with different mass or number of neutrons. In conclusion, to check easily if you will encounter questions about isotopes next time, look first if the given examples have the same element but do not have the same number of neutrons.
Isotopes are atoms of the same element (same atomic number) but with a different number of neutrons (different atomic mass). Choice A and C are incorrect due to they are not same elements, regardless of the mass amount. Choice D is correct in that they are the same element, but they have the same mass so they are the same not an isotope.