Explanation: The squelch circuit's primary function is to mute the receiver's audio output when the incoming signal strength falls below a user-adjustable threshold. This prevents the constant hiss and static that would otherwise be heard in the absence of a strong, usable signal. If the squelch circuit fails, it loses its ability to suppress this background noise. Consequently, the receiver will continuously produce audio (static/hiss) even when no sufficiently strong input signal is present, precisely matching the described symptom. * **A) AGC (Automatic Gain Control)** adjusts amplifier gain to maintain consistent audio levels for varying signal strengths, but it doesn't mute the receiver. * **B) The BFO (Beat Frequency Oscillator)** is used for demodulating SSB and CW signals; its failure would prevent proper reception of these modes, not cause continuous noise. * **D) The AF (Audio Frequency) amplifier** boosts the audio signal for the speaker/headphones. If it failed, there would be no audio output at all, which is the opposite of the problem.