Which symbol indicates that an aircraft is grounding or unsafe/unfit for flight?

The symbol that indicates an aircraft is grounded or deemed unsafe/unfit for flight is the letter "X." This marking signals that the aircraft is not airworthy, and it serves as a crucial visual cue for maintenance personnel and flight crews to ensure that necessary inspections or repairs are completed before the aircraft can be returned to service.

Using "X" allows for clear communication regarding the aircraft's status, which is essential for safety in aviation operations. Other options do not carry the same significance regarding airworthiness; for example, a forward slash "/", dashes "---", and the letter "M" do not represent grounded conditions or unfit statuses and may represent different statuses or designations that are not directly related to airworthiness. Thus, "X" effectively communicates that the aircraft is unfit for flight, ensuring that safety protocols are upheld.