Explanation: The discriminator is the correct answer because it is a circuit specifically designed to produce an output voltage that varies with the frequency of its input signal. In an Automatic Frequency Control (AFC) system, this voltage is used to detect any drift in the receiver's local oscillator frequency relative to the desired Intermediate Frequency (IF). If the IF drifts, the discriminator generates an error voltage (positive for too high, negative for too low, or zero for correct frequency) which is then fed back to adjust the local oscillator, maintaining precise tuning. A peak detector (A) extracts the amplitude of a signal, not its frequency deviation. A crystal mixer (B) converts RF signals to IF signals by combining them with a local oscillator, but it doesn't generate control voltage for AFC. A second detector (C) is a general term for the stage that demodulates the IF signal to extract information, but the *specific* circuit developing the AFC voltage by sensing frequency shifts is the discriminator.