The correct answer is B) by varying the input frequency of the voltage to the motor. In a diesel-electric propulsion system, the speed of the AC propulsion motor is controlled by varying the frequency of the voltage supplied to the motor. This is typically done using a variable frequency drive (VFD), which adjusts the frequency of the electrical input to the motor, allowing for precise control of the motor's rotational speed. Increasing the frequency increases the motor speed, while decreasing the frequency reduces the speed. The other options are incorrect because they do not accurately describe the primary means of controlling the speed of an AC propulsion motor in a diesel-electric system. Decreasing the motor voltage (A) or increasing the current to the motor (C) would not provide the same level of speed control as varying the input frequency. Increasing the motor voltage (D) may increase the motor's power output, but it would not directly control the speed.