Which transponder mode provides pressure altitude information?

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

Answer: (B)

Pressure altitude information is carried by Mode C. This mode adds an altitude field to the transponder reply, reporting the aircraft’s altitude in feet above standard atmosphere, which ATC uses for vertical separation.

Mode A only sends a four-digit squawk code for identification and contains no altitude data. Mode S provides more advanced data and selective addressing, including various data blocks, but the classic, straightforward source of pressure altitude in replies is Mode C. Mode M is an older, less-used option that doesn’t provide the standard pressure altitude information in the same way as Mode C.