Which transponder mode provides altitude information such as pressure altitude?

• A. Mode A
• B. Mode C
• C. Mode S
• D. Mode M

Answer: (B)

Altitude information is carried by the transponder mode that adds altitude data to the reply. A transponder in Mode A only sends a four‑digit squawk code for identification, without any altitude. Mode C, on the other hand, includes altitude encoding in its reply, providing the aircraft’s pressure altitude in 100‑foot increments so air traffic control can determine vertical position. Mode S offers more advanced data and capabilities beyond basic altitude reporting, but the standard way to obtain altitude information in this context is through Mode C. Mode M is primarily a maintenance mode and does not provide live altitude data for surveillance.