The correct answer is A) adjusting the input voltage to the motor. The speed of a DC propulsion motor is directly proportional to the voltage applied to it. By adjusting the input voltage, you can control the speed of the motor. This is a common and effective way to control the speed of DC motors. The other options are not the primary means of speed control for DC propulsion motors. Option B, the use of static power converters, is more commonly used for AC motors. Option C, the use of a load-commutated inverter, is typically employed for synchronous motors. Option D, adjusting the output frequency, is not applicable for DC motors as they do not operate based on frequency.