The correct answer is A) improper positioning of brush rigging outside the neutral plane. When the brush rigging is not properly positioned within the neutral plane of the motor, it can result in excessive sparking at the brushes. This is because the brushes will be contacting the commutator at a point where the magnetic field is not at its weakest, leading to arcing and increased wear on the brushes and commutator. The other options are incorrect because: B) varying loads during maneuvering can cause sparking but is not the primary cause, C) reversed main field polarity would cause the motor to run in reverse, not increase sparking, and D) reversed armature polarity would prevent the motor from running at all.