Which of the following is NOT a mode of IFF?

The correct response identifies that Mode 5 is not a traditional mode of Identification Friend or Foe (IFF) as recognized in the context of historical IFF systems. The IFF system is primarily designed to differentiate between friendly and enemy aircraft by using transponders.

Mode 1, Mode 2, and Mode 3 are all established modes used in older IFF systems. Mode 1 is primarily used for military aircraft identification, while Mode 2 is used for individual aircraft identification. Mode 3, also known as "civilian mode," is widely employed for air traffic control and is commonly used in commercial aviation.

Mode 5, on the other hand, is a more recent development intended to enhance identification capabilities and improve security against spoofing and other threats. Although it is indeed an IFF mode, it is not classified in the same way as the foundational modes are, hence its distinction in this context. This reflects how developments in military systems can add complexity to established nomenclature and clarify which modes are foundational versus those that represent advancements in technology.