Explanation: SITOR (NBDP) is a narrow-bandwidth digital mode, typically requiring a very precise and narrow Intermediate Frequency (IF) filter for proper reception and decoding. A 500 Hz IF filter is designed for exactly this purpose, isolating the narrow digital signal from noise and adjacent interference. If the 500 Hz IF filter becomes defective, the radio's ability to process these narrow signals is severely impaired or lost, preventing reception in SITOR (NBDP) mode. However, voice and SSB modes use a much wider IF filter (e.g., 2.4 kHz or 2.7 kHz). The failure of the *separate* 500 Hz filter would not affect the functionality of the wider SSB filter, allowing you to still hear signals in voice/SSB mode. * **A) The phase locked loop (PLL) circuit has failed:** A PLL failure would affect frequency stability and reception across *all* modes, not just one. * **C) The R.F. amplifier has failed:** An RF amplifier failure would significantly reduce or eliminate reception for *all* modes equally. * **D) The B.F.O. circuit has failed:** A BFO failure would prevent proper demodulation of *all* SSB and CW-type signals (including SITOR), making voice/SSB unintelligible or nonexistent.