Answer:
D. found in nature.
Explanation:
Isotope is any of two or more forms of a chemical element, having the same number of protons in the nucleus, or the same atomic number, but having different numbers of neutrons in the nucleus, or different atomic weights. There are 275 isotopes of the the 81 stable elements, in addition to over 800 radioactive isotopes, and every element has known isotopic forms. Isotopes of a single element possess almost identical properties. An isotope of an element is just a version of that element with a particular number of neutrons. Some numbers are stable, some are not; the ones that aren't shoot particles out at extremely high speeds ("radiation").
Since radioisotopes generally have the same chemistry as their stable counterparts (since neutrons play almost no role in chemistry), they'll get wherever other atoms of that element would get. Iodine, for example, accumulates in your thyroid gland - so do radioactive forms of iodine, which can then cause thyroid cancer by irradiating it from the inside.
Also because they're chemically almost indistinguishable, they're almost impossible to separate out by any normal means. It's like giving someone a giant bin of golf balls, some of which are 1% heavier than the other golf balls but are otherwise exactly the same, and saying "okay, sort these". Except the golf balls are atoms and the bin is the size of a small country.
