At what vertical level does prohibited airspace begin?

Prohibited airspace begins at the surface. This means that there are no altitudinal restrictions that keep aircraft from entering this airspace, making it critical for pilots to be aware of such designations right from ground level. The establishment of prohibited airspace is usually due to national security interests or safety concerns, such as areas surrounding military bases or sensitive locations.

Understanding that prohibited airspace starts from the surface highlights its strict nature; all flights, regardless of altitude, are not permitted in these areas. This mandates attention from pilots and air traffic control to ensure compliance and maintain safety within the national airspace system.

The other choices suggest various altitude thresholds, which do not apply in this context. They imply that there is a safe altitude above which flight may be permitted, whereas the reality is that entry into prohibited airspace is forbidden at any altitude starting from the ground level.