What is the altitude requirement for Class A airspace?

Class A airspace is defined as airspace from 18,000 feet above mean sea level (MSL) up to and including 60,000 feet MSL. This category of airspace is characterized by its strict operational rules and requires that all aircraft operating within it be under instrument flight rules (IFR). Therefore, the altitude requirement for Class A airspace begins at 18,000 feet ASL and goes up to 60,000 feet, making the correct answer the one indicating that the minimum altitude is 18,000 feet ASL and above.

This is critical for maintaining high-level air traffic, ensuring that all planes are operating safely under controlled conditions, as Class A airspace is primarily used by commercial jet traffic and requires communication with air traffic control at all times.