During the cruise checklist, if you see the MASTER WARNING light with "MANIFOLD FAIL," what is the first action you should take?

When you encounter a "MASTER WARNING" light accompanied by a "MANIFOLD FAIL" indication during the cruise checklist, the appropriate first action is to press the TRIM AIR switchlight off.

The MANIFOLD FAIL message typically indicates a potential issue with the air distribution system, specifically involving the trim air system. By turning off the TRIM AIR, you effectively isolate that portion of the environmental control system, which may stabilize the situation and address the failure indication. This action is a standard initial response to help mitigate any adverse effects that might result from the manifold failure.

Other options, while also part of the broader troubleshooting and response procedures, are not the immediate primary actions to take in response to a MANIFOLD FAIL indication. The focus should first be on the trim air system to ensure continued safety and proper operation of the aircraft.