The correct answer is D) a dirty commutator. Excessive sparking at the brushes of a running DC motor is an indication of a dirty or worn commutator. The commutator is responsible for transferring electrical current from the stationary brushes to the rotating armature windings. If the commutator surface is dirty or uneven, it can cause increased resistance and arcing at the brush-commutator interface, leading to excessive sparking. The other answer choices are incorrect because: A) water vapor absorption would not directly cause sparking, B) increased brush capacity does not relate to the issue, and C) normal operation of a DC motor should not produce excessive sparking.