The correct answer is C) by means of slip rings. In a conventional AC generator, the direct current from the exciter output is passed through the windings of the main field rotor by means of slip rings. Slip rings allow the transfer of electrical current from the stationary part of the generator (the stator) to the rotating part (the rotor) without the use of a commutator, which would be susceptible to wear and tear. The slip rings provide a continuous electrical connection between the exciter and the main field windings, enabling the generation of the necessary magnetic field for the production of alternating current. The other options are incorrect because: A) a commutator is used in DC generators, not AC generators; B) a rotating bar magnet is not involved in the process of transferring exciter output to the main field rotor; and D) minimizing the danger of arc over is not the primary purpose of using slip rings in this context.