Answer is Carbon-14
Radioactive isotopes have half lives. Half life is the time taken by the radioactive isotope to reduce its initial amount by half.
Oxygen-16 and Carbon-12 are stable isotopes of elements O and C respectively. Hence, they do not have half-lives. But Carbon-14 and Uranium-238 are radioactive isotopes. Among them Carbon-14 has relatively short half-life as about 5730 years while Uranium-238 has a long half-life as about 4.5 billion years.
