Question

1. which part of the atom is being changed in nuclear fusion?

Explanation:

In nuclear fusion, atomic nuclei (containing protons and neutrons) of light atoms combine to form a heavier atomic nucleus. The nucleus (protons and neutrons) undergoes change as new nuclei are formed, while electrons are not directly involved in the fusion process (which occurs in the atomic nucleus region).

Answer:

The nucleus (specifically, atomic nuclei combine to form a new, heavier nucleus; protons and neutrons within the nucleus are rearranged/changed as lighter nuclei fuse into a heavier one).