Explanation: Vacuum tubes typically have high output impedance, often in the kilohms range, while headphones are low impedance (e.g., 75 ohms). **A) By using an impedance matching transformer:** This is the most common and efficient method. An audio output transformer is specifically designed to step down the high impedance of the vacuum tube's plate circuit to match the low impedance of the headphones, ensuring maximum power transfer. **B) Connect them as a cathode bias resistor:** This method is less common but viable, especially for low-power applications. A cathode resistor provides self-bias for the tube. The audio signal current (AC component) flows through the cathode, and if the headphones are in this path, they will reproduce the sound. The impedance of the headphones (75 ohms) would also contribute to the tube's bias voltage. Since both methods A and B can be used to connect low impedance headphones to a vacuum tube output, option C is correct.