Explanation: SITOR (SImplex Teletype Over Radio) in ARQ (Automatic Repeat reQuest) mode is an interactive, error-correcting digital communication method. When operating in ARQ, data is transmitted in specific blocks. These blocks are indeed three characters long, consisting of two information characters and one service character. The service character facilitates error detection and acknowledgment. If a block is not correctly received, the transmitting station will automatically retransmit it, ensuring reliable data delivery. Option B is incorrect because ARQ relies on acknowledgments for each block or small group of blocks, not solely at the end of the message, to enable real-time error correction. Option C is incorrect; SITOR ARQ's primary error correction mechanism is retransmission, not time diversity. Option D is incorrect as Forward Error Correction (FEC) is a non-interactive, simplex mode, where the receiver corrects errors without requesting retransmissions, making it unsuitable for interactive communication.