An unscheduled report taken when certain criteria have been observed describes a?

The correct choice is SPECI, which refers to a special report issued to update weather information in between routine aviation reports. It is generated when there are significant changes in weather conditions that affect aviation, such as a sudden drop in visibility or the onset of thunderstorms. This type of report is unscheduled and is generated only when certain specific criteria are met, making it essential for pilots and air traffic controllers to have the most current and relevant information for safe flight operations.

Other choices, such as METAR, refer to routine hourly weather reports, AIRMET indicates weather advisory information for light aircraft, and TAF provides a forecast for aviation weather conditions over a specified time. Each of these serves a different purpose in the context of aviation weather reporting. SPECI stands out as the report specifically addressing unscheduled changes based on certain observed criteria, making it the right answer.