Question

1. what happens to nuclei during fission?

they decay.
they are combined into heavier nuclei.
they split into smaller nuclei.
they remain the same and release energy.

Explanation:

Nuclear fission is a process where a heavy nucleus splits into smaller nuclei. Decay is a different nuclear process (like alpha, beta decay). Combining into heavier nuclei is fusion. Remaining the same and releasing energy doesn't describe fission.

Answer:

They split into smaller nuclei.