Mode C transponder capabilities include which of the following?

• A. Identifies aircraft; Position
• B. Pressure altitude information
• C. Ground speed only
• D. Vertical speed only

Answer: (B)

Mode C adds altitude information to the transponder's reply. Specifically, it reports pressure altitude, which ATC uses to determine vertical separation. That makes the option describing pressure altitude information the correct one. The other data—ground speed, vertical speed, or identity with position—aren’t provided by Mode C in its standard response. Ground speed and vertical speed aren’t part of the typical Mode C reply, and position isn’t inherent to Mode C (it’s associated with other systems such as Mode S/ADS-B).