In nuclear reactors, controlling the chain reaction is crucial. Neutrons released during fission must be absorbed to prevent an uncontrolled reaction. Boron rods are used as control rods because boron has a high neutron capture cross-section, meaning it efficiently absorbs neutrons. By inserting or withdrawing these rods, operators can precisely regulate the reactor's power output.

Graphite and water are moderators, which slow down neutrons to sustain fission, not absorb them. Coolant removes heat. Therefore, the correct answer is the boron rod.

Final Answer: Boron rod