Answer: -
A. An uncontrolled nuclear chain reaction is most likely to multiply exponentially.
Explanation: -
An nuclear chain reaction produces neutrons. These neutrons then can start new nuclear fissions. In a controlled nuclear fission, the number of neutrons allowed to start new fission is controlled.
In an uncontrolled reaction, the neutrons start fission very fast. These also start new nuclear fission, releasing more neutrons.
Thus. An uncontrolled nuclear chain reaction is most likely to multiply exponentially.