Nuclear Reactor is made to use the energy which is released due to nuclear fission reaction.
These chain reactions are very fast and they are used with some components which can control these reactions otherwise these reaction will start with high rate and could not be controlled after that.
In order to control these reactions we need to control the high energy particles which can stimulate these reactions
These are neutrons which act as catalyst of these nuclear reactions and these reactions can be control by controlling the numbers of neutrons
These neutrons can be absorbed by some mechanism and the rods which are used to absorb these high energy neutrons are known as control rods.
So correct answer must be control rods
