The correct answer is D) incorrect brush grade, pressure or position. Excessive DC propulsion motor brush sparking is typically caused by issues with the brushes, such as an incorrect brush grade, improper brush pressure, or improper brush positioning. The brushes need to make proper contact with the commutator to transfer current efficiently, and any deviation from the recommended specifications can lead to excessive sparking. In contrast, the other options (a dark chocolate colored commutator, brushes positioned in the neutral plane, or a concentric commutator) are not direct causes of excessive brush sparking.