The correct answer is (d) Liquid sodium.
Liquid sodium is used as a coolant in the fast breeder reactor at Kalpakkam because it has a number of advantages over other coolants. It is a good conductor of heat, which means that it can remove heat from the reactor core very effectively. It is also a good neutron moderator, which means that it slows down the neutrons produced by the fission reaction so that they can be captured by the fuel rods. This helps to maintain the chain reaction and produce more power.
Carbon dioxide, heavy water, and sea water are all possible coolants for nuclear reactors, but they each have their own disadvantages. Carbon dioxide is not a good conductor of heat, so it would not be able to remove heat from the reactor core as effectively as liquid sodium. Heavy water is a good conductor of heat, but it is also a good neutron absorber, which means that it would absorb neutrons from the fission reaction and reduce the power output of the reactor. Sea water is a good conductor of heat, but it is also corrosive, which means that it would damage the reactor components.
Liquid sodium is therefore the best coolant for the fast breeder reactor at Kalpakkam because it has the best combination of properties. It is a good conductor of heat, a good neutron moderator, and it is not corrosive.