Explanation: A discriminator's role is to convert frequency deviations into a proportional voltage, typically used for Automatic Frequency Control (AFC). In microwave systems, klystrons are often employed as local oscillators or exciters. The unique characteristic of a klystron is that its oscillation frequency can be precisely controlled by varying the voltage applied to its repeller (or reflector) electrode. Therefore, applying the discriminator's error voltage to the klystron's repeller allows the system to automatically adjust and stabilize the klystron's frequency, keeping it centered or locked to a desired frequency. Options B and C, the grids of IF or RF amplifiers, are involved in signal amplification, not the frequency control of an oscillator. A magnetron (D) is a high-power microwave generator, but it is not typically frequency-controlled by a discriminator in an AFC loop; klystrons are the more common device for this application.