What are the vertical dimensions of Class A airspace?

The correct choice indicates that Class A airspace extends from 18,000 feet above sea level (ASL) to 60,000 feet flight level (FL600). This specific range is critical for ensuring safe and efficient control of air traffic, particularly for high-altitude flights.

Class A airspace is modeled to cover high-altitude operations, where most of the jet routes exist, providing a controlled environment for aircraft operating above 18,000 feet. The upper limit of 60,000 feet ensures that the airspace can accommodate a variety of flight operations, including those performed by both commercial and military aircraft. Encompassing this range also aids in managing separation requirements between various classes of users.

In contrast, the other options do not accurately represent the established vertical limits of Class A airspace as outlined by aviation regulations.