You are correct, the answer is B) open shunt field coil. In a DC motor, the speed is primarily controlled by the strength of the magnetic field produced by the shunt field coils. If the shunt field coil becomes open, it will reduce the magnetic field strength, allowing the motor to run faster than its designed speed. This is because with a weaker field, the armature can spin faster before the counter-EMF generated by the armature stops the acceleration. The other answer choices are incorrect because: A) a reversed commutating pole would not cause the motor to run faster, C) an overload would cause the motor to slow down or stall, and D) an open armature coil would cause the motor to stop running entirely.