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 energy through nuclear fusion. In its core, hydrogen nuclei combine (fuse) to form helium (a heavier nucleus). Uranium - related processes (fission or fusion) do not occur in the Sun, and hydrogen does not undergo fission to form a heavier nucleus in the Sun's context.

Answer:

through the fusion of hydrogen nuclei into a heavier nucleus