Final answer:
During a fission chain reaction, the number of neutrons increases.
Step-by-step explanation:
In a fission chain reaction, the correct fact is that the number of neutrons increases. When a heavy nucleus such as uranium-235 is split, it releases multiple neutrons. These neutrons can then collide with other nuclei, inducing further fission reactions and creating a chain reaction. This chain reaction results in the release of a significant amount of energy.