What is the vertical extent of Class A airspace in the U.S.?

The correct answer is that Class A airspace in the U.S. extends from 18,000 feet MSL (Mean Sea Level) to and including Flight Level 600 (FL600). This vertical limit allows it to encompass a substantial portion of the upper flight levels used by high-altitude commercial and military aircraft, where air traffic control manages air navigation and ensures separation between aircraft.

Class A airspace is specifically designated for IFR (Instrument Flight Rules) operations only, meaning that all aircraft must be equipped for instrument flight and must operate under the control of Air Traffic Control (ATC). This higher classification and its defined vertical limits are essential for maintaining organized air traffic, especially in busy airspace regions where the potential for conflicting routes increases significantly.

The other options do not correctly reflect the vertical limits established for Class A airspace. Specifically, options that suggest altitudes below 18,000 feet or an upper limit of FL350 do not align with the established parameters of Class A airspace, reinforcing the significance of understanding airspace classifications and their dimensions.