Question

how does the sun release nuclear energy?
through the fusion of a uranium nucleus into smaller nuclei
through the fusion of hydrogen nuclei into a heavier nucleus
through the fission of a uranium nucleus into smaller nuclei
through the fission of hydrogen nuclei into a heavier nucleus

Explanation:

The Sun releases nuclear energy through nuclear fusion. Nuclear fusion is the process where lighter nuclei combine to form a heavier nucleus. In the Sun, hydrogen nuclei (protons) fuse together. Uranium is involved in nuclear fission (splitting of a heavy nucleus), which is not the process in the Sun. Also, hydrogen nuclei undergo fusion, not fission.

Answer:

through the fusion of hydrogen nuclei into a heavier nucleus