Explanation: A dynamotor essentially combines a DC motor and a DC generator on a single shaft. Its output voltage can be regulated by controlling either the motor's operation or by modifying the output directly. 1. **Changing the speed of the motor (Option A):** The output voltage of a DC generator is directly proportional to its rotational speed and the strength of its magnetic field. By varying the input voltage to the dynamotor's motor section, its speed changes. This, in turn, alters the speed at which the generator section rotates, directly affecting the generated output voltage. 2. **Using a series resistance in the output line (Option B):** Placing a resistor in series with the dynamotor's output will cause a voltage drop across that resistor, as per Ohm's Law (V=IR). This effectively reduces the voltage delivered to the load. While this method primarily *reduces* voltage and dissipates energy as heat, it is a practical way to regulate (decrease) the available output voltage. Since both methods are viable for adjusting or controlling the output voltage of a dynamotor, option C, encompassing both A and B, is the correct answer.