What element represents the final product of sequential fusion stages within the core of the most massive stars before their collapse?

Iron.

In stars significantly larger than the Sun (greater than about eight solar masses), the core temperature and pressure continue to rise after hydrogen exhaustion, enabling further stages of stellar nucleosynthesis. Once helium is fused into carbon and oxygen, the core continues to contract and heat until it can initiate the fusion of these heavier elements into successively heavier products, such as neon and magnesium. This progression continues until the core successfully fuses elements all the way up to iron. Iron is the critical endpoint because fusing iron consumes energy rather than releasing it, making it energetically impossible for the star to generate further outward thermal pressure through fusion at that stage, leading inevitably to core collapse.