What does Mode 3/A provide to Air Traffic Control (ATC)?

Mode 3/A is a specific transponder code used in aviation that primarily provides altitude information to Air Traffic Control (ATC). When an aircraft is equipped with this transponder and responds to an ATC interrogation, it transmits a unique identifiable code along with the aircraft's current altitude information. This allows ATC to track the aircraft’s movements and ensure safe separation from other air traffic.

In this context, the option that focuses on positional information may relate to the general understanding that transponders help ATC identify where an aircraft is located. However, it is the altitude data that is most critical for ATC in managing air traffic effectively. Other options such as speed and direction data are generally provided by different instruments or systems and not solely through Mode 3/A, while weather updates are not part of what Mode 3/A transmits. Thus, the core function associated with Mode 3/A is indeed altitude information, which is essential for maintaining safety and order in the airspace system.