Which reaction primarily occurs in stars, including our sun?

In stars, including our sun, the primary reaction that occurs is nuclear fusion. This process involves the combination of lighter atomic nuclei to form heavier nuclei, which releases a tremendous amount of energy. In the case of our sun, hydrogen nuclei (protons) fuse together under extreme temperatures and pressures to form helium nuclei. This fusion process is the source of the sun's energy, sending heat and light to the solar system.

Nuclear fusion is critical for the lifecycle of stars, as it sustains them and allows them to shine for billions of years. The energy produced from fusion reactions also plays a vital role in balancing the gravitational forces pulling the star inwards, maintaining its stability.

The other reactions listed, such as nuclear fission, radioactive decay, and beta decay, do not primarily occur in stars. Fission involves splitting heavy atomic nuclei into lighter nuclei and usually takes place in man-made reactors rather than in stellar environments. Radioactive decay and beta decay involve the transformation of unstable nuclei but do not account for the primary energy production in stars. Therefore, nuclear fusion is the correct answer regarding the primary reaction occurring in stars.