Final answer:
The key difference is that fission splits heavy nuclei into smaller parts, while fusion combines light nuclei into larger ones, with fusion releasing more energy and contributing to Earth's internal heat.
Thus option D.is correct answer.
Step-by-step explanation:
The principal distinction between nuclear fusion and nuclear fission is their respective processes and outcomes. Fission is the division of an atom into two or more smaller atoms, and fusion is the merging of two or more small atoms into a larger one.
Fusion reactions release 3 to 4 times more energy than fission per gram of fuel and use lighter, more abundant elements, such as hydrogen. Fission reactions rely on heavy elements like uranium. While fission can be used in nuclear reactors and weapons, and does indeed create the chain reactions used in reactors, controlled fusion remains a significant challenge, though it is the reaction that powers stars and has the potential to be a clean and virtually limitless energy source if controlled effectively.
Fusion is the significant contributor to the thermal energy within Earth's interior and is a naturally occurring process in stars, including our Sun. As for the options provided, the correct statement that describes how nuclear fusion and nuclear fission differ is: D. Only fusion produces the thermal energy that makes Earth's interior hot.