Which of these can be used to control the output level of a generator?

The correct answer is the application of a small DC voltage to the field winding of the generator. This method is essential for controlling the output level of a generator because the field winding generates the magnetic field required for electricity production. By varying the amount of DC voltage applied to the field winding, the magnetic field strength can be adjusted, which directly influences the generator’s output voltage.

Increasing the voltage to the field winding enhances the strength of the magnetic field, leading to a higher output voltage from the generator, whereas decreasing the voltage results in a weaker magnetic field and lower output voltage. This is an effective and commonly used method in various types of generators to maintain a steady output despite changes in load conditions, ensuring compatibility with the demands of the connected electrical system.

Using a small AC voltage applied to the armature does not effectively control the output level, as this would primarily induce an alternating current but not provide a consistent means to regulate the overall output voltage of the generator. A large DC voltage at the output could potentially damage the generator or create unsafe conditions rather than allow for proper control. An external resistor could be used to limit current under certain circumstances, but it would not provide a reliable method to control the generator's output voltage effectively.