Answer : Option C) A meltdown.
Explanation : When a nuclear fission reaction is conducted, and if it loses its control it will produce many neutrons which will be difficult to bring under control; for this purpose the nuclear reactors are built with the control rods which absorb the extra neutrons and help the reaction to be in control.
If this is not done, it may result in complete meltdown of the reactor as neutrons have high energy in them.