Explanation: An open circuit in the transmission line (B) means the antenna is disconnected from the transceiver's receiver input. Without a connection to the antenna, no radio frequency (RF) signals from the air can reach the receiver's internal circuitry for processing and demodulation. Therefore, no matter how strong the signals are outside, or how well the audio amplifier stage functions (evidenced by "strong AF output," likely amplified background noise), no actual radio signals will be heard. * **A) The squelch sensitivity control is set too high:** This would prevent weaker signals from being heard, or even mute all signals if set excessively high. However, the "strong AF output" implies the audio section is active, which contradicts a fully squelched receiver. * **C) The final stage of the AF amplifier circuit has failed:** If the AF (Audio Frequency) amplifier failed, there would be no, or very weak, audio output. This directly contradicts the statement "strong AF output." * **D) The power amplifier stage in the receiver has failed:** Receivers primarily have RF and IF (Intermediate Frequency) amplifiers, not "power amplifiers" in the sense of transmitters. If an RF or IF amplifier stage failed, signals wouldn't be heard. However, an open transmission line is a more fundamental and external point of failure that completely isolates the receiver from *all* incoming signals, making it the most likely and comprehensive cause for hearing *no signals on any frequency*.