The correct answer is D) they reduce the voltage applied to the motor during the starting period. Autotransformer starters are used with larger polyphase induction motors to reduce the voltage applied to the motor during the starting period. This helps limit the high starting current that would otherwise flow through the motor windings, which could potentially damage the motor. By reducing the voltage, the autotransformer starter allows the motor to gradually build up torque and speed, rather than applying full voltage and current immediately, which could overload the motor. The other options are incorrect because: A) autotransformers do not allow for stepping up voltage, they only step it down; B) they are not a backup for voltage regulation, but rather a primary means of managing the starting process; and C) they reduce, not increase, the voltage for starting the motor.