The correct answer is "armature". The armature is the rotating component of an AC generator that is responsible for producing the changing magnetic field which generates the electrical energy. The other options mentioned are also parts of a power plant or electrical system, but not specifically part of an AC generator. The commutator is part of a DC generator, the turbine is part of a power plant that may drive the generator, and a transformer is used to step up or step down the voltage generated by the AC generator.
