The correct answer is B) a canvas wiper. The commutator face of a DC propulsion motor should be cleaned using a canvas wiper, as this is a soft, non-abrasive material that will not damage the delicate commutator surface. Abrasive materials like emery cloth, abrasive dressing stones, or sandpaper can potentially create grooves or unevenness on the commutator, which can lead to increased sparking and premature wear. Using a canvas wiper is the recommended method to gently clean and maintain the commutator face without causing any harm to the motor.