Explanation: This scenario points to a problem specific to voice/SSB reception, while other modes remain functional. Let's analyze the options: * **A) The B.F.O. circuit has failed.** The Beat Frequency Oscillator (BFO) is crucial for demodulating both SSB voice and SITOR (NBDP) signals, as SITOR is typically transmitted as AFSK (Audio Frequency Shift Keying) over an SSB carrier. Without a working BFO, neither mode would be intelligible. Since you can hear SITOR, the BFO is likely functional. * **B) The audio amplifier is defective.** If the audio amplifier failed, no sound from *any* mode would be heard through the speaker or headphones. Since SITOR is audible, the audio amplifier is working. * **D) The R.F. amplifier has failed.** The RF amplifier is at the front end of the receiver. If it failed, all incoming signals would be severely attenuated or completely absent. Since SITOR signals are being heard, the RF amplifier is likely functional. * **C) The 2.8 kHz filter has become defective.** Voice SSB signals require a bandwidth of approximately 2.4 to 2.8 kHz for proper intelligibility. This is usually provided by an IF (Intermediate Frequency) filter. SITOR (NBDP), while transmitted over an SSB carrier, relies on a much narrower bandwidth (typically a few hundred Hz) for its tone pairs. If the 2.8 kHz filter, which is specifically designed for voice SSB, becomes defective (e.g., shorted, open, or severely detuned), it would effectively block or severely distort the wider voice signal, making it inaudible or unintelligible. However, a radio often has narrower filters (e.g., 500 Hz or 300 Hz) that are automatically switched in for digital modes like SITOR, or the SITOR signal might still be discernible through a defective wider filter if only a narrow passband within it is still working. Therefore, a failure of the specific SSB voice filter explains why voice is lost while the narrower SITOR signals can still be heard.