Which ground-to-air symbol means 'unable to proceed'?

The symbol that indicates "unable to proceed" in ground-to-air communications is represented by the letter "X." This designation is used by pilots and ground personnel to effectively communicate situations where a person or vehicle is unable to continue moving or operating as intended. Such communication is crucial for ensuring safety and coordination in aviation operations, allowing for prompt responses and necessary support.

Understanding this symbol is essential for navigating emergency situations or changes in plans, where clear signals of inability to continue are necessary for safety and operational efficiency. The other choices represent different messages or commands within ground-to-air communications, but "X" specifically denotes the inability to proceed in a direct and recognized format.