Final answer:
Iron is the heaviest element that can be created in energy-producing fusion reactions, as it is the most tightly bound and stable of all nuclei.
Step-by-step explanation:
During the process of nuclear fusion, light nuclei such as hydrogen combine to form heavier nuclei, releasing a significant amount of energy.
This fusion, which powers stars such as the Sun, primarily produces helium from hydrogen. In stars, the fusion of light elements can continue until heavier elements are formed. The heaviest element which can be created in energy-producing fusion reactions, within a star, is iron, the most tightly bound of all nuclei. Beyond iron, fusion processes consume energy instead of producing it, marking the end of energy-generating fusion in star lifecycles.