What mode is used only by military and is a secure method to identify aircraft?

The correct answer, which refers to Mode 4, is a secure identification method specifically designed for military use. Mode 4 utilizes encrypted signals to ensure that only authorized military aircraft can be identified, thus enhancing security against potential threats or unauthorized access. This secure communication is critical in military operations, where safety and accurate identification are paramount.

In contrast, the other modes serve different purposes and audiences. Mode 3, for example, is primarily used for civilian air traffic control and does not incorporate the same level of security features as Mode 4. Mode 2 is also a military identification mode but is not secure; it is often utilized for situational awareness without the encryption protocols that Mode 4 offers. Mode S, while providing more advanced capabilities compared to older systems, is focused on civilian air traffic and does not include secure communications specific to military operations.

Thus, Mode 4 is uniquely tailored for military purposes, ensuring secure identification, which is why it is the right answer to the question.