What does a transponder code do in aviation?

The correct answer highlights the primary function of a transponder code, which is to identify an aircraft to radar systems. When an aircraft is equipped with a transponder and the appropriate code is set, it sends out a signal that includes specific information about the aircraft, such as its identity, altitude, and sometimes additional data. This allows air traffic control and other radar-equipped systems to differentiate one aircraft from another and track their movements on radar displays.

The other options focus on aspects that a transponder does not provide. While navigation in poor visibility is important, it is achieved through instruments and procedures, not via transponder codes. An aircraft's destination is not indicated by the transponder; this information is communicated through other means, such as flight plans. Lastly, transponders do not provide weather updates; instead, weather information is typically received from radar systems or meteorological services through other communication systems.