Final answer:
Nuclear fuel has a significantly higher energy density than fossil fuels due to the nature of nuclear reactions, which involve changes in atomic nuclei and release more energy per reaction than burning fossil fuels.
Step-by-step explanation:
The correct answer to the question comparing nuclear energy and fossil fuels is option D: Nuclear fuel has a higher energy density than fossil fuels. While both nuclear fuel reactions and fossil fuel reactions release heat that can be used to do work (thus eliminating options A and B), the key distinguishing factor lies in their energy densities. Nuclear reactions, including both fission and fusion, release a significantly greater amount of energy compared to the chemical reactions involved in burning fossil fuels such as coal, petroleum, and natural gas. This is because nuclear reactions involve changes in the nucleus of an atom, which holds considerably more energy than chemical reactions involving electron arrangements.