Which transponder mode provides altitude reporting to the secondary radar system?

The choice that provides altitude reporting to the secondary radar system is Altitude Reporting. This mode enables the transponder to transmit aircraft altitude information, which is essential for air traffic control and helps in tracking the aircraft's vertical position. In this mode, the transponder picks up the altitude data from the aircraft's altimeter and relays it to ground radar systems, allowing controllers to maintain separation between aircraft and enhance overall air safety.

Basic mode typically does not provide altitude information; it transmits a simple identification code. Emergency mode is used to signal distress, but it does not inherently include altitude reporting. Squawk mode is a general term for any of the transponder functions but is not specific to altitude reporting. Therefore, Altitude Reporting is the correct mode for providing the necessary altitude data to the secondary radar system.