At what altitude does the transition from Class E airspace to Class A airspace occur?

The transition from Class E to Class A airspace occurs at 18,000 feet Mean Sea Level (MSL). Class A airspace is designated for high-altitude operations and begins at 18,000 feet MSL, extending up to flight level 600 (FL600) or 60,000 feet MSL. This is an important point for pilots to understand, as operations within Class A airspace require adherence to specific regulations, including Instrument Flight Rules (IFR) and having an appropriate flight plan.

Class E airspace serves a different purpose, typically covering lower altitude areas and extending upward to the base of Class A airspace, which is why recognizing this specific transition altitude is crucial for safe navigation and compliance with airspace regulations. The other altitude options do not align with the established transition point from Class E to Class A, making them incorrect for this question.