Answer: A. A chain reaction occurs when uranium atoms split, releasing nuclear energy.
Explanation: Nuclear fission reactions are those nuclear reactions in which heavier nuclei splits into two or more lighter nuclei. In these reactions, neutrons are released with a large amount of energy.
General equation for nuclear fission reactions is given by
[tex]_{92}^{235}\textrm{U}+_0^1n\rightarrow _{98}^{236}\textrm{U}\rightarrow _{56}^{139}\textrm{Ba}+_{36}^{94}\textrm{Kr}+3_0^1n[/tex]
The released 3 neutrons further react with remaining uranium atoms and thus a chain reaction sets up.