The correct answer is D) the strength of the rotating magnetic field. The amount of RMS voltage induced in the windings of an AC generator is determined by the following factors: the number of turns in the armature coils, the strength of the rotating magnetic field, and the rotational speed of the generator. Of these, the strength of the rotating magnetic field is the only factor that can vary during generator operation, as it can be affected by changes in the generator's excitation system or the load conditions. The other factors, such as the number of armature coil turns and the rotational speed, are generally fixed or controlled parameters. The other answer choices are incorrect because they represent fixed or controlled parameters that do not change during normal generator operation.