The correct answer is B) varying the strength of the rotating magnetic field. The voltage output of an AC generator is directly proportional to the strength of the rotating magnetic field. By varying the strength of the magnetic field, you can control the voltage output of the generator. This is the most practical way to control the voltage, as it allows for fine-tuning and adjustment of the output. The other options are not as practical or effective for controlling voltage. Varying the speed of the rotating field (A) would be difficult to control precisely. Changing the number of windings (C) would require physical modifications to the generator. Varying the power factor of the load (D) would not directly control the generator's output voltage.