U-235 differs from U-238 in that U-235 is able to produce a fission chain reaction. U-238 is the most common isotope of uranium, with the U-235 isotope constituting less than 1% of natural sources. Uranium enrichment is the process by which U-235 is separated out, and it is this product that is most commonly used to manufacture nuclear weapons. Pu-239 is produced from U-238 and can be used for nuclear weapons as well, but Pu-239 is not an answer choice.