Question

a chain reaction that is caused by nuclear fission
a chain reaction that is caused by nuclear fusion
a decrease in energy that is caused by nuclear fission
an increase in energy that is caused by nuclear fusion

Explanation:

The diagram shows uranium-239 splitting into smaller nuclei and releasing neutrons, which then trigger further splitting of uranium-235 nuclei. This sequential splitting of heavy atomic nuclei is nuclear fission, and the self-sustaining sequence is a chain reaction. Nuclear fusion involves combining light nuclei, which is not depicted here.

Answer:

a chain reaction that is caused by nuclear fission