Which of the following is a characteristic of Class A airspace?

Class A airspace is defined to encompass the area from 18,000 feet mean sea level (MSL) up to and including 60,000 feet MSL, and it is characterized primarily by its requirement for all aircraft to operate under Instrument Flight Rules (IFR). In order to enter and operate within Class A airspace, pilots must obtain an ATC clearance, which is a critical aspect of maintaining order and safety in high-altitude environments where air traffic is more congested and varied. This requirement ensures that air traffic controllers can effectively manage the flow of aircraft, reducing the risk of collisions and improving overall airspace efficiency.

The other choices pertain to different classifications or situations that do not apply to Class A airspace. For example, Class A airspace is not designated for Visual Flight Rules (VFR) only and visual separation is not applied consistently as it is in lower airspace classes. Additionally, Class A airspace does not start at the surface or extend only to 3,000 feet; it begins at 18,000 feet. Understanding these specifics is crucial for navigating the complexities of airspace classifications as outlined by the Federal Aviation Administration (FAA).