Explanation: SITOR (NBDP) ARQ (Automatic Repeat reQuest) mode is an error control protocol designed for reliable data transmission over radio. In this mode, the transmitting station (e.g., ship) sends data in blocks, typically groups of characters. Each character uses a 7-unit code that has inherent error-detecting capabilities, often referred to as a constant ratio code, which ensures a specific number of mark and space bits. The receiving station (e.g., shore) checks these incoming character groups for errors based on this constant ratio (similar to a parity check). * If a group is received correctly (no errors detected, "parity is OK"), the receiving station sends a positive acknowledgment (ACK) signal. This tells the ship station to transmit the *next* group of characters. * If an error is detected, the receiving station sends a negative acknowledgment (NACK), often called an "RQ" (ReQuest) signal, prompting the ship station to retransmit the *same* group. Option D accurately describes the initial phase of a successful transmission: the ship sends a group, the shore checks for errors, and if none are found, the process proceeds (implying an ACK and transmission of the next group). Options A and B are incorrect because SITOR ARQ does not initially send characters *twice* as its primary error control mechanism; it relies on error detection and *retransmission on request*. Option C is incomplete and implies a request is always made, which isn't how ARQ efficiently handles correctly received data.