Explanation: SITOR (Narrow-Band Direct-Printing) is a digital mode that uses Frequency Shift Keying (FSK). To generate the FSK signal for transmission, the transceiver's internal modem or digital signal processor synthesizes specific audio tones (often called mark and space tones). These tones are then fed into the SSB modulator to generate the RF signal. If the 1615 kHz audio oscillator (or similar tone generator) responsible for producing these digital mode tones is not working, the transceiver cannot generate the necessary audio signal for SITOR, even if the rest of the SSB transmit path is functional. Voice/SSB operation relies on the microphone input directly, not on a specific internal audio oscillator for signal generation, so it would remain unaffected. Let's look at why the other options are incorrect: * **A) The second I.F. amplifier has become defective:** An IF amplifier is part of the main signal path for both transmit and receive, regardless of the mode. If it were defective, neither voice/SSB nor SITOR would work properly. * **B) The voltage controlled oscillator is defective:** A VCO is critical for frequency generation and stability. A defective VCO would likely affect all modes, causing major tuning or transmission problems across the board. * **C) The 2.8 kHz filter has become defective:** This filter is typically for SSB voice. If it were defective, voice/SSB performance would be severely degraded. While SITOR signals might pass through an SSB filter, a failure of this filter would primarily impact voice and would not selectively disable SITOR while leaving voice/SSB perfect.