Which airspace encompasses federal airways below 18,000 feet?

Class E airspace is the correct answer because it is the classification of airspace that includes federal airways, which are designed for navigation and are part of the National Airspace System. Federal airways operate from the surface up to 18,000 feet, and they typically consist of routes established for the use of aircraft navigating under instrument flight rules (IFR).

In this context, Class E airspace is designated to accommodate IFR and visual flight rules (VFR) traffic while providing the necessary controlled environment to ensure safe separation and efficient air traffic management. This airspace can extend from the surface or from various designated altitudes up to, but not including, 18,000 feet.

The other classifications of airspace mentioned serve different purposes. Class G airspace, for example, is uncontrolled airspace that exists below the base of Class E airspace and has no specific air traffic control services. Class D airspace surrounds airports with operational control towers and is typically lower than Class E. Prohibited airspace is designated for security reasons and is not meant for any aviation operations.