The correct answer is D) Incorrect brush grade, pressure or position. Excessive brush sparking in a DC propulsion motor can be caused by improper brush selection, installation, or maintenance. The brush grade, pressure applied to the brushes, and their positioning relative to the commutator are all critical factors that can contribute to excessive sparking if not properly addressed. Incorrect brush grade may not match the commutator material, leading to increased friction and sparking. Improper brush pressure can allow the brushes to bounce or chatter against the commutator. Brushes that are not positioned in the neutral plane of the motor will also experience increased sparking. The other answer choices are incorrect because: A) Brushes positioned in the neutral plane should minimize sparking, not cause excessive sparking. B) The color of the commutator is not a direct indicator of excessive sparking. C) The shape of the commutator (concentric or otherwise) does not directly cause excessive brush sparking.