The correct answer is C) Starting of the prime mover. The prime mover, such as an engine or turbine, provides the rotational torque that causes the AC generator to turn and produce electrical current. When the prime mover is started, it applies the necessary rotational force to the generator's rotor, which then generates the alternating current. The other options are incorrect because: A) Providing current to the field from an external source is not what generates the initial rotational torque to turn the generator. B) Residual magnetism in the armature does not directly create the rotational torque to spin the generator. D) Residual magnetism in the field also does not directly produce the rotational force to turn the generator.