Explanation: Automatic Frequency Control (AFC) systems in receivers are designed to maintain accurate tuning by compensating for frequency drift. In a superheterodyne receiver, the incoming RF signal is mixed with the local oscillator (LO) signal to produce a fixed intermediate frequency (IF). If the LO's frequency drifts, the IF will shift, degrading reception. An AFC circuit senses this IF shift (often by detecting a deviation from the center frequency of the IF amplifier) and generates a correction voltage. This voltage is then applied to the local oscillator (frequently a voltage-controlled oscillator or VCO), adjusting its frequency to bring the IF back to the desired value and keeping the receiver precisely tuned to the transmitted signal. The magnetron (A) is a high-power microwave generator, typically used in transmitters (e.g., radar, microwave ovens), not for receiver tuning. The IF amplifier stage (B) amplifies the fixed IF signal but does not vary the receiver's tuning frequency. A cavity duplexer (D) is used in transceivers to allow a single antenna for both transmitting and receiving; it plays no role in varying receiver tuning frequency.