What obstacle clearance and navigation signal coverage does the Minimum Sector Altitudes provide?

The Minimum Sector Altitudes (MSA) are designed to provide a minimum level of obstacle clearance and ensure adequate navigation signal coverage within a specified area around a navigation facility. The criteria established for MSAs ensure that aircraft maintain a safe altitude that is clear of obstacles while also providing a reliable signal from the navigation facility.

In this context, the correct choice indicates a clearance of 1,000 feet within a 25 nautical mile (NM) radius of the navigation facility. This level of clearance enhances safety by allowing for sufficient vertical space above any terrain or obstacles that may be present in that area. Additionally, a 25 NM radius is an appropriate distance to ensure that aircraft operating in this airspace can receive reliable navigation signals, as this distance typically encompasses the area where signals from the facility remain strong and usable.

The other options suggest either lower clearances at similar or shorter radii, higher clearances with decreased radii, or combinations that do not align with established MSA standards. Hence, the first choice accurately reflects the MSA guidelines, ensuring both safety from obstacles and effective navigation coverage.