Which category of anti-air systems is described as highly mobile, portable, and difficult to detect?

The selection of Man-Portable Air Defense Systems, known as MANPADS, accurately reflects the characteristics described in the question. MANPADS are designed to be lightweight and easily transported by individual soldiers, making them highly mobile. Their portability allows them to be deployed quickly in various environments, including urban areas or rugged terrain.

Additionally, these systems are typically low in radar signature and designed to operate with a minimal logistical footprint, which contributes to their difficulty in detection. Their compact size and the nature of their launch mechanisms make them an effective tool for ground forces, allowing for quick engagements against aerial threats while remaining inconspicuous.

In contrast, other categories such as Air Defense Artillery (ADA) and Tactical SAMs usually involve larger systems that may require more complex support structures and can be more easily identified due to their size and operational setup. Active radar missiles, while technically sophisticated, also rely on radar guidance systems that can potentially give away their position, thus lacking the stealth advantage associated with MANPADS.