Explanation: In ARQ (Automatic Repeat reQuest) communications, especially when interacting with automated systems like Coast Stations, specific control characters are used to manage the protocol and terminate commands. The `+` character commonly serves as a command terminator or an action indicator, signaling the automated station to process the preceding command string. This convention helps the automated system accurately parse commands, preventing premature execution or misinterpretation of data. While `ENTER` (carriage return) is typical for terminating lines in local computer interfaces, radio protocols often employ distinct control characters like `+` for command execution to maintain precise control over the communication link and ensure proper protocol flow over the air. Other options like `GA+?` are procedural signals or queries within a communication, not general command terminators. `END` is a word, not a single character command terminator.