Final answer:
Plutonium is a highly radioactive byproduct of uranium fission in nuclear reactors, known for its long half-life, toxicity, and uses in military weapons, as well as its storage challenges due to its persistent radioactivity.
Step-by-step explanation:
The statement that most likely describes plutonium is that it is a highly radioactive substance mostly formed in nuclear reactors as a byproduct during the fission of uranium. Specifically, when U-235 undergoes fission, additional neutrons are released, some of which combine with U-238 nuclei to eventually form plutonium-239 through a series of beta decay processes involving uranium-239 and neptunium-239. Plutonium is known for its long half-life and its alpha (a) emission, which can be particularly damaging to human health if inhaled or ingested, making it one of the most toxic substances known. While used in military weapons due to its small critical mass, the management of plutonium presents a challenging storage problem because of its long half-lives, ranging from thousands to hundreds of thousands of years.