Answer:
Helium-4
Step-by-step explanation:
Nuclear fusion is the process by which two light nuclei joins to form a single heavy nucleus and during this process, a large amount of energy is released.
In case of Sun or other stars in the universe this fusion results in the production of the havoc energy which powers the heat and light of the star.
Inside the sun's core hydrogen atoms undergo fusion resulting in the formation of helium thus releasing a lot of energy which is the reason for sun's light and heat. This process requires the extreme conditions of high temperature and pressure to take place as both the nuclei have a positive charge and thus they repel each other. So in extremely high temperature and pressure the nuclei gains very high speed, so much that before they start repelling each other they just hit each other, thus performing a successful fusion.