The correct answer is D. Carbon.
Carbon is the most common material used for commutator brushes because it is a good conductor of electricity and has a low coefficient of friction. It is also relatively inexpensive and easy to machine.
Mica is an insulating material that is sometimes used in commutator brushes to prevent arcing. However, it is not a good conductor of electricity, so it is not used as the primary material for commutator brushes.
Copper is a good conductor of electricity, but it has a high coefficient of friction. This means that it can wear down the commutator more quickly than carbon.
Cast iron is a strong material that is sometimes used in commutator brushes to withstand the wear and tear of the commutator. However, it is not a good conductor of electricity, so it is not used as the primary material for commutator brushes.