What limits the height of obstructions between decision altitude and runway threshold in an ILS?

The Glidepath Qualification Surface is the correct choice because it specifically defines the area around the ILS glidepath where obstructions must be limited to ensure a safe and smooth approach to the runway. This surface ensures that any objects or terrain features do not interfere with the glidepath, which is typically set at a specific angle to guide aircraft down to the runway. By maintaining this surface clear of significant obstructions, the safety of the instrument approach is preserved, allowing for reliable navigation and descent as the aircraft approaches the decision altitude.

Other surfaces, such as the Obstacle Clearance Surface, relate to ensuring that there is adequate clearance for aircraft operations but do not specifically govern the obstructions affecting the glidepath. The Height Control Surface, while it suggests certain height limitations within specific airspaces, does not directly address the particular requirements of the glidepath, nor is it widely recognized as a formal surface in the context of ILS approaches. The Runway Safety Area focuses on the area surrounding the runway itself for safety purposes, rather than the specific glidepath definition critical during the approach phase.