Answer : To confine a fission reaction
Explanation :
Usually a controlled fusion reactor will be used to monitor fusion reactions, and NOT fission reactions. As a sustained controlled nuclear fusion reactor will absorb neutrons and form a bigger nuclei than before. But whereas, a fission reaction, will emit neutrons from the uranium nucleus and continue to do the same until a stable nucleus is formed. It will keep on dividing and producing more neutrons. This can be controlled using control rods to minimize the rate of production of neutrons.