Question

which best describes a radioactive isotope?
an atom is unstable.
an atom is stable.
there is only one other isotope.
there are many isotopes.

Explanation:

A radioactive isotope (radioisotope) is an atom with an unstable nucleus, which means it will undergo radioactive decay to become more stable. A stable atom does not undergo radioactive decay, so the option "An atom is stable" is incorrect. The number of other isotopes (whether one or many) is not the defining characteristic of a radioactive isotope; the key is the instability of the atom's nucleus. So the correct description is that the atom is unstable.

Answer:

A. An atom is unstable.