What basic requirement must be fulfilled for IFF identification?

The correct answer, challenge and response, is fundamental for Identification Friend or Foe (IFF) systems. IFF technology operates on the basis of a two-way communication process wherein an identified aircraft or system responds to an interrogating signal. This response helps to distinguish friendly forces from potential threats. The challenge is usually an interrogation signal transmitted by radar or another detecting system, which can then be answered by the aircraft's IFF transponder, further enhancing situational awareness.

This process is crucial as it effectively allows military personnel and automated systems to verify identities in real-time, minimizing the chances of misidentifying friendly forces as enemies during operations. By confirming a pilot's or aircraft’s identity through this challenge and response mechanism, it significantly contributes to operational security and effectiveness.

The other options involve important operational aspects but do not fulfill the core requirement of IFF identification. Altitude verification ensures aircraft are at safe flight levels, radar reflection pertains to the detection capability of aircraft, and flight plan approval involves administrative procedures rather than the active identification process itself.