Explanation: In an Automatic Frequency Control (AFC) system, the discriminator is a critical component that detects frequency deviations. Its purpose is to generate an error signal proportional to the difference between an input frequency and a reference frequency, or between two input frequencies. In many radar or microwave receiver AFC systems, the signal from a **magnetron** (often the received echo signal, or a sample of the transmitted signal) is mixed with the local oscillator (LO) signal, which is commonly generated by a **klystron**. This mixing process produces an Intermediate Frequency (IF). The discriminator then compares this IF to a desired reference IF. If the IF deviates, the discriminator generates a voltage that adjusts the klystron's frequency, effectively "locking" the klystron's frequency to maintain a constant IF, thereby tracking the magnetron's frequency. Therefore, the discriminator is comparing the frequency relationship between the magnetron and the klystron. * **B) PRR generator and magnetron:** The PRR (Pulse Repetition Rate) generator controls the timing of the radar pulses, not the magnetron's RF operating frequency. * **C) Magnetron and crystal detector:** A crystal detector rectifies RF signals but does not compare frequencies or generate a reference. * **D) Magnetron and video amplifier:** A video amplifier processes low-frequency, post-detection signals and is not involved in RF frequency comparison.