Answer:
a. have an excess number of neutrons.
Step-by-step explanation:
Every atomic nucleus have a specific neutron/proton ratio that makes them stable. Any nucleus with a neutron/proton ratio that differs from the stability ration will become unstable. An unstable nucleus will split up and emit small particles of matter and/electromagnetic ionizing radiation. This makes an atom radioactive.
Therefore, an atom with excess number of neutrons is a radioactive isotope.