The correct characterization is:
Isotopes of an element vary in the quantity of neutrons they possess. Certain unstable isotopes are utilized in the fields of nuclear medicine and nuclear power generation.
This statement accurately reflects that isotopes, which are variants of an element, differ in their neutron count. In nuclear medicine, certain unstable isotopes (radioisotopes) are used for imaging and treatment, while in nuclear power generation, unstable isotopes (such as uranium-235) are key for nuclear fission reactions.