Final answer:
The Federal Aviation Administration mandates that airplanes maintain a distance of five miles from another plane at the same altitude when less than 40 miles from the radar antenna, so the correct answer is 5 miles (Option A).
Step-by-step explanation:
To determine the minimum separation required between an aircraft and obstructions depicted on the radar display when less than 40 miles from the antenna, we need to follow air traffic control regulations set by the Federal Aviation Administration. This situation involves understanding distances and safety measures within air traffic management, a branch of physics dealing with the movements and locations of objects in airspace, such as airplanes.
The Federal Aviation Administration (FAA) requires that each airplane must maintain a distance of five miles from another plane at the same altitude, as well as a vertical separation of 2,000 feet above and below another aircraft (or 1,000 feet if the altitude is less than 29,000 feet). Therefore, the correct answer to the question is: Separate aircraft from obstructions depicted on the radar display when less than 40 miles from the antenna by 5 miles (Option A).