The right answer is Uranium (especially U-235).
Nuclear fission is the phenomenon by which a heavy atomic nucleus (that is, formed of a large number of nucleons such as uranium, plutonium, etc.) is split into two or more lighter nuclides.
This nuclear reaction is accompanied by the emission of neutrons (generally two or three) and a very important energy release (≈ 200 MeV (megaelectronvolt) per fissioned atom, compared to the energies of the chemical reactions which are the order of the eV per atom or molecule reacting).
