The correct answer is B) provide reduced voltage for starting. Autotransformers used with autotransformer starters on large AC motors function to provide a reduced voltage during the motor's starting process. This lower starting voltage helps limit the high inrush current that would occur if the motor was started at full line voltage, protecting the motor and electrical system. The autotransformer steps down the voltage to a more appropriate level for starting, then switches to full line voltage once the motor is up to speed. The other options are incorrect because: A) increased voltage is not needed for starting, C) speed control is not the purpose of the autotransformer, and D) reduced voltage, not increased torque, is the primary function.