The correct answer is A) the strength of the rotating magnetic field. The voltage induced in the stator windings of an AC generator is primarily dependent on the strength of the rotating magnetic field. This is in accordance with Faraday's law of electromagnetic induction, which states that the induced voltage is proportional to the rate of change of the magnetic flux linking the stator windings. The other answer choices are not the primary factors determining the induced voltage. The number of field poles (B) affects the frequency of the induced voltage, but not its magnitude. The cross-sectional area of the stator windings (C) affects the resistance, but not the induced voltage. The number of brushes (D) is relevant for DC generators, but not for the operation of AC generators.