Explanation: The output voltage of an AC generator is directly proportional to the strength of the magnetic field and the speed at which the armature conductors cut through that field. In a separately-excited generator, a DC field current flows through the field winding to create the magnetic field. **B) The amount of field current:** By varying the DC field current, you change the strength of the magnetic field. A stronger magnetic field induces a higher voltage in the armature windings (Faraday's Law of Induction), thereby controlling the generator's output voltage. **A) The armature:** The armature is the component where the voltage is induced. Its physical construction (number of turns, winding type) is fixed and determines its inherent characteristics, but it is not a control mechanism for the output. **C) The brushes:** Brushes provide electrical contact between the stationary external circuit and the rotating armature windings (via slip rings in an AC generator). They conduct the generated current but do not control the voltage generation process. **D) The exciter:** The exciter is a smaller DC generator or rectifier that *provides* the DC field current for the main generator. While the exciter is the *source* of the field current, the *amount* of current it supplies is the actual control parameter. The exciter itself is the supply, not the variable control.