What is the primary process responsible for the energy generation in main-sequence stars?

The primary process responsible for energy generation in main-sequence stars is the nuclear fusion of hydrogen into helium. In a main-sequence star, the core's extremely high temperatures and pressures enable hydrogen nuclei (protons) to overcome their natural repulsion due to the electromagnetic force. This process occurs through a multi-step reaction, commonly referred to as the proton-proton chain reaction, where multiple hydrogen nuclei fuse to eventually form helium.

During this fusion process, a significant amount of energy is released in the form of gamma rays, which eventually makes its way to the surface of the star and is emitted as light and heat. This energy generation process maintains the balance between the gravitational collapse of the star and the outward pressure from the energy produced in the core, allowing the star to remain stable.

The other processes mentioned in the options are either not relevant in the context of main-sequence stars or occur under different conditions. For instance, nuclear fission of heavier elements generally takes place in massive stars or in artificial reactors and is not a primary energy source in main-sequence stars. Gravitational contraction can contribute to energy generation during a star's formation, but once it reaches the main sequence phase, nuclear fusion becomes the dominant process. Similarly, chemical reactions involving carbon