Which interface allows the Sentinel to exchange air track information between sensor mode and external track sources through digital communications?

The correct answer is that the Low-Level Air Picture Interface (LLAPI) is specifically designed to enable the Sentinel system to exchange air track information, particularly in the context of operating in lower altitude airspace where it may need to coordinate with external track sources. This interface facilitates digital communication, allowing real-time data exchange that ensures effective air defense operations and enhances situational awareness.

LLAPI is particularly relevant when dealing with comprehensive air operations where information from various sensors and external sources must be integrated seamlessly. Understanding this capability is crucial for operators, as it directly impacts their effectiveness in managing air defense resources and responding to aerial threats. The other options, while they may sound relevant, focus on different aspects or levels of air picture information and do not specifically address the digital communications aspect as it relates to the Sentinel system's needs in exchange with external track sources.