What occurs during thermonuclear fusion?

During thermonuclear fusion, hydrogen atoms undergo a process where they combine under extreme temperatures and pressures to form helium. This reaction is the fundamental process that powers stars, including our sun. In conditions of high temperature, the hydrogen nuclei (protons) move at such high speeds that they overcome their natural repulsion due to the electromagnetic force and get close enough for the strong nuclear force to take effect, allowing them to combine.

This fusion process releases a tremendous amount of energy in the form of light and heat due to the mass-energy equivalence principle articulated by Einstein's equation, E=mc². The mass of the resulting helium nucleus is slightly less than the combined mass of the original hydrogen nuclei, and this lost mass is converted into energy.

The other options do not pertain to thermonuclear fusion. Oxygen combining with hydrogen to form water is a different chemical reaction known as combustion. The combination of carbon atoms into carbon dioxide represents another type of chemical reaction, typically associated with oxidation, and the splitting of helium atoms, which would involve nuclear fission instead of fusion, does not accurately describe the process of thermonuclear fusion.