Which of the following statements concerning SITOR communications is true?

In SITOR (Simplex and Telex Operations Real-time) communications, the operation is primarily focused on efficient message transmission, which involves the use of Automatic Repeat reQuest (ARQ) protocols. The correct statement highlights that ARQ message transmissions are made in data groups consisting of three-character blocks. This is significant because ARQ is designed to ensure that transmissions are accurate and can be retried if an error is detected or if acknowledgment is not received. The use of three-character blocks helps streamline the communication process and allows for quick and efficient handling of messages.

Understanding the context, ARQ is fundamentally about error correction and reliable delivery, which aligns with the function of transmitting in these specific group sizes. The other statements do not accurately capture the characteristics of SITOR communications—frequency specifics, the nature of messages allowed, and the acknowledgment process are different from the implications of sending three-character blocks, reflecting the robustness of the ARQ system utilized in SITOR.