Which substance is used as Coolant in nuclear reactors to remove heat?

The correct answer is Option B — Cold water and Liquid Sodium. The most common coolants are ordinary water (H2O) or heavy water (D2O) in water-cooled reactors and liquid sodium in fast breeder reactors. The coolant absorbs heat and carries it to heat exchangers where steam is produced to drive turbines. Option A is incorrect: Graphite is used as a moderator to slow down neutrons not as a coolant. Cadmium is used in control rods to absorb neutrons. Neither absorbs and transfers heat efficiently as a coolant. Option C is incorrect: Boron is used in control rods. Lead is used as radiation shielding. Neither is used as a primary coolant for heat removal in reactors. Option D is incorrect: Helium and argon are noble gases. While helium is used as coolant in some high-temperature gas-cooled reactors, argon is not used as nuclear reactor coolant. The primary coolants for most reactors are water and liquid sodium.