The correct answer is D) changing the generator engine speed. In a DC diesel-electric drive system, the speed of the DC propulsion motor is primarily controlled by adjusting the speed of the generator engine. By increasing or decreasing the generator engine speed, the generator's output voltage and frequency are correspondingly changed, which directly affects the speed of the DC propulsion motor. This is the primary method of speed control in such systems. The other options are incorrect because they do not directly control the propulsion motor speed. Changing the generator field excitation (option A) or the motor field excitation (option B) can affect the motor speed, but they are not the primary means of speed control. Changing the polarity of the generator field (option C) would not be a practical way to control the motor speed.