(a) atom or molecule: From these choices, the more massive one will have to be the molecule. This is because a molecule is composed of two or more atoms making it weigh more.
(b) an isotope or ion: For these choices, the more massive element would be the isotope. This is because an isotope will have more neutron than compound while that of the ion will only have additional or lacking electron. An electron is very much lighter than neutrons.