Explanation: A motor-generator's output voltage, and thus its potential output power, is directly proportional to the strength of the magnetic field within the generator. This magnetic field strength is controlled by the current flowing through the generator's field windings. To increase the output, a stronger magnetic field is required, which means increasing the current in the field windings. A rheostat placed in series with the generator field circuit controls this current. By *decreasing* the resistance of this rheostat (option A), more current is allowed to flow through the field windings. This stronger field current creates a more intense magnetic field, inducing a higher voltage and thus increasing the overall output from the generator. Conversely, increasing the resistance (option B) would reduce the field current, weakening the magnetic field and decreasing output. Option C is incorrect as output is readily adjustable, and option D describes an improper and disruptive procedure for simply increasing output.