What defines Class A airspace?

Class A airspace is defined as the airspace that extends from 18,000 feet Mean Sea Level (MSL) up to and including Flight Level 600. This airspace is not visually depicted on sectional charts, as it's predominantly utilized by high-altitude aircraft flying under instrument flight rules (IFR). The specification of starting at 18,000 feet signifies that it is designed to accommodate the high traffic density associated with jet aircraft and provides the required air traffic control services and separation necessary for safety.

In terms of the other options, while Class A airspace does not start at 10,000 feet MSL, it encompasses the entire nation, but it does not surround major airports, which may fall under different classes of airspace such as Class B, C, or D. Understanding that Class A is specifically assigned for the operation of IFR flights at high altitudes helps clarify its distinction from the other airspace classifications.