The reaction exemplified in the diagram describes nuclear fission because an atom is splitting into two large fragments of comparable mass.
What is nuclear fission?
Nuclear fission in radioactivity is the process by which the nucleus of a large atom splits into smaller particles.
According to the diagram in this question, uranium atom with mass number 235 splits into barium (141) and krypton (92) atoms.
Therefore, the reaction exemplified in the diagram describes nuclear fission because an atom is splitting into two large fragments of comparable mass.
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