The correct answer is B) Starting of the prime mover. The prime mover, such as a diesel engine or a turbine, provides the rotational torque that causes the AC generator to turn and produce electrical power. When the prime mover starts up, it applies the necessary torque to the generator's rotor, which then induces a voltage in the stator windings through electromagnetic induction. This starting of the prime mover is the key method used to provide the rotational torque to the generator. The other options are incorrect because: A) Residual magnetism in the field would not be sufficient to start the generator. C) Providing current to the field from an external source may be used to control the generator's voltage, but it does not provide the initial rotational torque. D) Residual magnetism in the armature does not generate the rotational torque needed to start the generator.