Final answer:
More massive stars fuse elements up to iron in their cores because iron is the most stable, and fusing iron requires energy rather than releasing it.
Step-by-step explanation:
The student is asking about the process by which more massive stars fuse elements in their cores. The process continues until elements are built up to iron. This happens because iron nuclei are the most tightly bound and stable of all elements, meaning that the fusion of iron does not release energy, but instead requires it. Therefore, the fusion process in stars ends with iron, making iron the correct answer (option b). Stars are not able to produce heavier elements like plutonium through fusion in their cores; such elements are formed during other processes like supernova explosions.