Which of the following is the correct separation distance for aircraft under radar separation at a distance of less than 40 miles from the radar antenna?

The correct separation distance for aircraft under radar separation when they are less than 40 miles from the radar antenna is 3 miles. This distance is important for maintaining safety during radar operations, as it ensures that aircraft are adequately separated to prevent collisions or incidents.

When aircraft are operating within 40 miles of the radar antenna, the requirement for a 3-mile separation allows air traffic controllers to provide consistent and effective management of air traffic, particularly in busy airspace. This standard is established based on factors such as radar accuracy, the speed of aircraft, and the need for safe buffer zones to account for uncertainties in navigation.

In contrast, the other options suggest smaller separation distances, which may not provide the same level of safety and could potentially lead to decreased situational awareness or increased risk of conflict between aircraft operating in close proximity.