The correct answer is C) 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 excessive arcing and sparking at the brush-commutator interface, which is not normal operation. The other answer choices are incorrect, as water vapor absorption would not cause excessive sparking, and increased brush capacity or normal operation would not result in this issue.