What should be used to delete any unused or unwanted symbols on a flight strip?

The most appropriate method for deleting any unused or unwanted symbols on a flight strip is to use an "X" or a minus sign. The reason this is the correct answer lies in the standard practices of flight strip management in air traffic control.

In ATC operations, clarity and quick understanding are crucial. The "X" serves as a clear indication that a piece of information is either negated or no longer relevant. This visual cue helps controllers quickly assess the flight strip without confusion. Similarly, the minus sign serves the same purpose and is widely recognized in various documentation practices as a notation for deletion or removal.

Other methods mentioned, such as using terms like "Erase" or "Cross," may be less visually distinct or standardized relative to an "X" or minus sign. While circles or squares might be used in some contexts to denote different statuses or highlights, they do not universally convey deletion. The mention of "Mark" or "Delete" is also less conventional in this context, as these terms are more verbal rather than representing a clear visual characteristic on the flight strip that denotes unused or unwanted symbols. Thus, "X" or "-" is the preferred and clear method for maintaining the integrity and clarity of flight strip information.