If your choices are the following: A.Neutrons released during a fusion reaction cause other nuclei to fuse.
B. Neutrons released during a fission reaction cause other nuclei to split.
C. Helium nuclei released during a fission reaction cause other nuclei to split.
D. Helium nuclei released during a fusion reaction cause other nuclei to fuse.
Then the answer is B. In fission, the nuclei is splitted and releases neutrons causing another chemical to form.