Which mode allows a transponder to automatically reply to a ground or airborne interrogation addressed to a specific aircraft?

• A. Mode A
• B. Mode C
• C. Mode S
• D. Both mode A and mode C

Answer: (C)

Mode S provides selective addressing, which means each aircraft’s transponder has a unique address and will only respond automatically to interrogations that are addressed to that specific aircraft. This lets ground or airborne interrogators target a single aircraft and receive its reply, without every nearby transponder shouting back. It also keeps traffic down and supports richer data exchange.

Modes A and C don’t include this selective addressing. They reply to interrogations in their range, which can lead to multiple aircraft responding and more clutter, and they don’t guarantee a response only to a particular aircraft. That's why the mode that fits the description is Mode S.