The correct answer is: D. as a moderator.
Graphite is a good moderator because it is a good neutron absorber. Neutrons are produced when uranium-235 atoms split, and they need to be slowed down in order to be captured by other uranium-235 atoms. Graphite slows down neutrons by scattering them. This is important because it allows the chain reaction to continue.
A lubricant is a substance that reduces friction between two surfaces. Graphite is not a good lubricant because it is a solid. Solids do not flow easily, and they can actually increase friction.
A fuel is a substance that is burned to produce heat. Graphite is not a good fuel because it does not burn easily. It is also not a good source of energy.
An insulator is a substance that does not conduct electricity. Graphite is a good insulator because it is a poor conductor of electricity. This is important because it prevents the flow of electricity through the reactor core.