The upper vertical limit of Class A airspace is?

Class A airspace is defined as the airspace from 18,000 feet MSL (Mean Sea Level) up to and including FL600 (Flight Level 600). This classification is intended primarily for high-altitude jet traffic, and aircraft operating in Class A airspace must be equipped with specific navigation and communication systems, as well as operating under Instrument Flight Rules (IFR).

The designation of FL600 as the upper vertical limit indicates that this airspace is designed for high-altitude operations where the majority of commercial jet traffic is found. By maintaining such a high altitude ceiling, Class A airspace ensures safe separation between the various types of aircraft navigating at these high flight levels, facilitating efficient use of the airspace while accommodating the operational requirements of air traffic control.

The other options do not correctly represent the upper vertical limit for Class A airspace, as they fall below the established boundary.