Correct Answer: (d) Boron
Solution: Boron - Boron rods are commonly used as control rods in nuclear reactors. Boron is a neutron absorber, meaning it can capture neutrons without undergoing fission itself. By inserting or withdrawing the boron control rods, the rate of the nuclear chain reaction within the reactor core can be controlled. Cadmium is another element often used for control rods. Uranium is the nuclear fuel, sodium can be used as a coolant in some reactors, and graphite is used as a moderator to slow down neutrons.
