Explanation: AGC (Automatic Gain Control) systems are designed to maintain a relatively constant audio output level despite variations in the strength of the incoming radio signal. It achieves this by sensing the signal strength and generating a control voltage, which is then fed back to adjust the gain of earlier stages in the receiver. The most critical component that *responds* to this control voltage to actually change the gain is a variable gain amplifier (VGA). If a VGA, particularly in the RF (Radio Frequency) or IF (Intermediate Frequency) stages, fails to vary its gain in response to the AGC control voltage, the AGC system cannot perform its function. Therefore, failure of a variable gain amplifier in the RF stage (or IF stage) directly prevents the AGC from controlling the receiver's overall gain. * **A) The B.F.O. circuit has failed:** The BFO (Beat Frequency Oscillator) is used for CW and SSB detection, but it is not part of the AGC gain control loop. * **B) The Voltage controlled Oscillator is inoperative:** A VCO is typically part of the receiver's tuning or frequency synthesis system, not the gain control system. * **C) The audio amplifier is defective:** A defective audio amplifier would affect the final sound output, but the AGC system would still attempt to adjust gain in earlier stages.