A classroom projector, which is turned on, displays a flashing LED code but does not...

The answer is C. Clean the air filter.

A flashing LED code on a projector usually indicates a problem with the projector’s hardware, such as the lamp, the fan, the temperature sensor, or the air filter. According to the web search results, some possible causes and solutions for this problem are:

Projector is too hot: Make sure that the vents and air filter are not clogged with dust or obstructed by nearby objects. Clean or replace the air filter. Make sure the environmental temperature is not too hot12.

Projector has overheated and turned off: Leave it turned off to cool down for five minutes. Make sure that the vents and air filter are not clogged with dust or obstructed by nearby objects. Clean or replace the air filter. If operating the projector at high altitude, turn on High Altitude Mode12.

Low air flow through the projector or air filter: Make sure that the vents and air filter are not clogged with dust or obstructed by nearby objects. Clean or replace the air filter12.