Question

what makes stars produce energy?
gravity
hydrogen
nuclear fusion
nuclear fission

Explanation:

Stars produce energy mainly through nuclear fusion. In the core of stars, under high - temperature and high - pressure conditions, hydrogen nuclei fuse together to form helium nuclei. This process releases a huge amount of energy. Gravity is the force that holds the star together and creates the conditions for nuclear fusion (but not the direct energy - producing mechanism). Hydrogen is the fuel for nuclear fusion in stars. Nuclear fission is the splitting of heavy nuclei (not the main process in stars).

Answer:

nuclear fusion