Explanation: Voice (SSB) transmissions require a wider intermediate frequency (IF) bandwidth, typically 2.4 to 2.7 kHz, which is provided by a general-purpose IF filter. SITOR (NBDP) is a narrow-band digital mode that greatly benefits from, and often requires, a much narrower IF filter, such as a 500 Hz filter, to effectively reject noise and adjacent interference for proper decoding. If the 500 Hz IF filter has failed, the transceiver cannot provide the necessary narrow bandwidth for optimal SITOR reception. Voice operation, using a different, wider IF filter, would remain unaffected. This specific failure explains why voice works but SITOR does not. Other options are incorrect because: B) A failed audio amplifier would prevent any audio from being heard, affecting voice as well. C) A failed phase-locked loop (PLL) would likely cause frequency instability or render the receiver completely inoperable for all modes. D) A failed R.F. amplifier would significantly impair reception across all modes.