Which statement regarding SITOR (NBDP) communications is true?

The statement regarding SITOR (NBDP) communications that is accurate is that in ARQ (Automatic Repeat reQuest), the "information sending station" transmits a block of three characters that the receiving station will subsequently acknowledge or request to be retransmitted. This defines a fundamental aspect of the ARQ method used in SITOR communications.

ARQ is designed for reliable data transmission, and when the sending station transmits a block of data, it waits for an acknowledgment from the receiving station. Each block of characters is structured so that if any errors occur during transmission, the receiving station can signal that the data block needs to be resent. This process ensures that communications are secure and accurate, as it allows for the detection and correction of transmission errors.

SITOR is primarily designed for digital communication at sea, where conditions can be variable, making the reliability of communication crucial. Thus, the mechanism of transmitting blocks of characters and relying on acknowledgments is a central aspect of the efficiency and functionality of this communication method.