Right option is (d) Uranium
For explanation: Uranium is the only fissionable nuclear fuel occurring in nature. It consists of 99.3% of 92U^238, 0.7% 92U^235 and small traces of 92U^234. Out of these isotopes only 92U^235 is used in fission reaction. Uranium is a very heavy metal which has been used as an abundant source of concentrated energy for 60 years.