The correct answer is C) notify the bridge that you will need to slow down to reduce the electrical load. Excessive sparking of the brushes on the main propulsion motor indicates an issue with the electrical system, likely caused by an overload. The first action should be to notify the bridge so they can reduce the ship's speed, which will in turn reduce the electrical load on the system. This will help mitigate the sparking and prevent potential damage to the motor. The other options are incorrect because decreasing the speed of the main generator (A) or the main generator voltage (B) may not directly address the issue with the propulsion motor. Decreasing the motor field current (D) could potentially worsen the problem by further reducing the motor's performance.