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

Using a small DC voltage applied to the field winding is an effective method for controlling the output level of a generator. In a DC generator, the field winding generates a magnetic field, and the strength of this magnetic field directly impacts the voltage produced by the generator. By adjusting the DC voltage level applied to the field winding, the magnetic field can be either strengthened or weakened. A stronger magnetic field leads to a higher output voltage, while a weaker field results in a lower output voltage. This principle allows operators to precisely control the output of the generator based on the requirements of the electrical load it is supplying.

The other choices describe conditions that do not effectively facilitate the adjustment of output voltage in the same manner. For instance, applying AC voltage to the field winding could introduce complications due to the alternating nature of AC, which is not conducive to generating a stable output in a DC generator. Applying a large DC voltage to the armature is inappropriate, as it can damage the generator and does not serve the function of regulating output voltage.