Explanation: Headphones operate by using an electromagnet (coils) interacting with a permanent magnet to vibrate a diaphragm. When connected directly in a vacuum tube's plate circuit, the steady DC plate current flows through the headphone coils, creating a constant magnetic field. The audio signal, which is an AC variation superimposed on this DC, then causes the magnetic field to fluctuate, moving the diaphragm to produce sound. For maximum sensitivity, the magnetic field created by the DC plate current must *add* to the field of the permanent magnet. This biases the diaphragm into an optimal position, allowing the superimposed AC audio variations to produce the strongest possible movement. If the polarity is reversed, the DC magnetic field opposes the permanent magnet, weakening the overall magnetic force and reducing the diaphragm's ability to respond effectively to the audio signal. Therefore, observing proper polarity (A) ensures the maximum conversion of electrical energy into sound. Option B is incorrect because the current should *add* to, not oppose, the field strength of the permanent magnet for maximum sensitivity. Since B is incorrect, C is also incorrect.