Thermonuclear fusion occurs when which of the following takes place?

Thermonuclear fusion is the process by which atomic nuclei combine at extremely high temperatures to form a heavier nucleus, releasing a tremendous amount of energy in the process. In stars like our Sun, this occurs primarily when hydrogen atoms collide with enough energy, overcoming their electrostatic repulsion, to fuse together and form helium. This fusion process is essential for the energy production that powers stars and influences the dynamics of the universe.

The other options provided do not accurately represent the process of thermonuclear fusion. The first option refers to the dynamics of clouds in space and how they might collapse under their own gravity to form stars, but does not pertain to the fusion process itself. The third option discusses the formation of proto-planets, which involves the accumulation of dust and gas rather than the fusion of atomic nuclei. Since thermonuclear fusion specifically describes the reaction of hydrogen atoms fusing to create helium, it is the correct answer in the context of this question.