Final Answer:
If the specified grade is not available, option **C. Yes, any grade of aviation fuel can be used.**
Step-by-step explanation:
In aviation, the use of alternative grades of fuel is generally permitted if the specified grade is not available. While it is ideal to use the recommended or specified grade for an aircraft, operational circumstances may necessitate the use of a different grade. Option C correctly acknowledges that any grade of aviation fuel can be used as an alternative if the specified grade is unavailable.
The flexibility to use different grades of aviation fuel is based on the compatibility of the alternative fuel with the aircraft's engine and the operational requirements. This flexibility ensures that aircraft can still operate safely and efficiently even when the specified grade is not readily accessible. Pilots and operators must adhere to guidelines and regulations to ensure that the selected alternative fuel is suitable for the specific aircraft and mission.
In summary, the aviation industry allows for the use of alternative grades of fuel if the specified grade is not available. This flexibility is essential for maintaining operational continuity and ensuring the safety of flights, provided that the selected alternative fuel meets the necessary criteria for compatibility and performance.