Explanation: SITOR (NBDP) Mode B utilizes Forward Error Correction (FEC) to enhance reliability, especially in the presence of noise like static crashes. **D) FEC reduces the error rate by transmitting each character twice.** This is correct. SITOR Mode B (FEC) achieves error reduction by transmitting redundant information. For instance, in a common implementation, each information bit is sent twice (or encoded with sufficient redundancy), allowing the receiver to reconstruct the original data even if one of the transmissions is corrupted by a static crash. This redundancy directly combats errors without requiring a return channel for retransmission. **A) Data flow rate depends on signal propagation.** This is incorrect. The nominal data flow rate of SITOR is fixed by its design. While poor propagation increases errors, it doesn't dynamically change the base data rate. **B) Idle characters are sent upon request.** This is incorrect. In SITOR Mode B (FEC), idle characters are continuously sent when no data is present to maintain synchronization, not upon request from the receiver. **C) Transmitter and receiver cannot synchronize.** This is incorrect. SITOR is specifically designed to achieve and maintain synchronization even in challenging conditions, using the continuous transmission of data or idle characters. While a severe static crash might momentarily disrupt it, the system is designed to quickly re-synchronize.