Of the listed factors that influence the amount of RMS voltage induced in the windings of an AC generator operating at a constant 60 Hz frequency, which factor is variable during generator operation?

Correct Answer: Option D — the strength of the rotating magnetic field

Induced RMS voltage is proportional to rotational speed, winding turns, and magnetic field strength. With constant 60 Hz frequency fixing rotational speed, only the exciter-controlled strength of the rotating magnetic field remains variable during normal operations.