The key difference between fission and a simple chemical reaction is that fission involves the splitting of atomic nuclei, while chemical reactions involve the rearrangement of electrons around atoms. So among the provided options, the most accurate answer would be:
Atoms are split into smaller pieces during fission.
This captures the fundamental nature of fission as a nuclear process, contrasting with the interactions occurring in chemical reactions. However, it is important to note that options about the conditions under which fission occurs (like involving radioactive forms of atoms) also provide relevant information about their distinction.