The correct answer is: D. Any of the above.
Severe sparking, rough commutator surface, and imperfect contact can all cause rapid brush wear in D.C. generators.
Severe sparking can cause rapid brush wear because it creates heat that can damage the brushes and the commutator. Rough commutator surface can cause rapid brush wear because it creates uneven contact between the brushes and the commutator, which can cause sparking and heat damage. Imperfect contact can cause rapid brush wear because it creates uneven current flow, which can cause heating and damage to the brushes and the commutator.
To prevent rapid brush wear, it is important to keep the commutator clean and free of dirt and debris. It is also important to keep the brushes in good condition and to replace them when they are worn.