Final answer:
An atom would be considered an isotope if it has a different number of neutrons or a different atomic mass from another atom of the same element, which corresponds to options B) and D) in the question.
Step-by-step explanation:
The number of protons in the nucleus of an atom determines its atomic number, which defines its element. For instance, all carbon atoms have six protons and are represented by the atomic number 6. The number of neutrons, however, can vary among atoms of the same element without changing the element itself. Atoms with the same number of protons but different numbers of neutrons are known as isotopes. Therefore, an atom will be considered an isotope if it has a different number of neutrons than another atom of the same element. The atomic mass of isotopes will differ due to the change in the number of neutrons as the atomic mass is the sum of protons and neutrons in the nucleus.
To directly answer the question, an element would be considered an isotope in the case given by option B) An atom with a different number of neutrons, as this is what differentiates isotopes from one another. Option D) An atom with a different atomic mass, would also be correct as it is inherently linked to the presence of a different number of neutrons.