Answer: In total 5 atoms have undergone nuclear fission reaction when the reaction gets uncontrolled.
Step-by-step explanation:
Nuclear fission reactions are one of the 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:

In the question, it is given that in a fissionable sample, when 1 atom undergoes fission 3 neutrons are released and we need to find the number of atoms when five additional nuclear fission reactions occur.
When in a nuclear fission reactions, 1 atom is undergoing fission.
Hence, in 5 nuclear fission reactions, (5 × 1) = 5 atoms will undergo fission.
Therefore, the correct answer is 5 atoms.