The correct answer is D) improper positioning of brush rigging outside the neutral plane. When the brushes are positioned outside the neutral plane of a DC propulsion motor, it can result in excessive sparking at the brushes. This is because the brushes are not aligned with the magnetic field, leading to a voltage difference across the brushes and increased arcing. Proper positioning of the brush rigging within the neutral plane is critical to minimize sparking and ensure smooth operation of the DC motor. The other answer choices are incorrect because: A) varying loads do not directly cause excessive sparking, B) reversed armature polarity would cause the motor to run in the opposite direction, and C) reversed field polarity would lead to an opposite magnetic field, but not necessarily excessive sparking.