The correct answer is D) magnetically by the rotating stator field. In a basic AC induction motor, the rotor currents are induced magnetically by the rotating magnetic field produced by the stator windings. The stator windings, when supplied with alternating current, create a rotating magnetic field that induces currents in the rotor, which in turn produces its own magnetic field. This interaction between the stator's rotating field and the rotor's induced magnetic field is what causes the rotor to spin, resulting in the motor's operation. The other options are incorrect because they do not accurately describe the mechanism by which the rotor currents are induced in an AC induction motor. External variable resistors, a bridge rectifier, and an armature and brushes are not the means by which the rotor currents are induced in this type of motor.