What type of surface begins at the edge of the primary surface and the approach surface, extending upwards at a 7:1 slope?

The type of surface that begins at the edge of the primary surface and the approach surface, extending upwards at a 7:1 slope, is known as the transitional surface. This slope is crucial for ensuring that aircraft can safely transition from the approach to the primary surface as they approach landing or take off. The transitional surface serves to enhance safety by providing an area that gradients off to help minimize obstacles and ensure a clear flight path.

In contrast to the transitional surface, the other options refer to different areas related to airport design and flight operations. The conical surface relates to a vertical extension that looks like a cone, typically used to define protected airspace above high structures. The horizontal surface is a flat plane that forms part of the airport's operational area. The instrument surface pertains to areas that are defined for instrument landing system approaches and contain specific requirements for obstacle clearance but does not specifically describe a surface that follows the 7:1 slope.