What transponder code should be used in the event of a hijacking?

The transponder code used in the event of a hijacking is 7500. This specific code is designated for communicating a hijacking situation to air traffic control and alerts them to the need for increased awareness and potential assistance. By setting the transponder to 7500, pilots can discreetly inform authorities of the emergency without overtly alarming the hijacker or compromising the safety of the passengers and crew.

In contrast, the other codes serve different purposes. Code 7600 is used to indicate a radio communication failure, meaning the pilot is unable to communicate with air traffic control but is not necessarily in any distress situation. Code 7700 is a general emergency code that signals any form of in-flight emergency, which may or may not pertain to hijacking. Finally, the code 1234 is a common practice code used for training or on the ground, and it does not convey any emergency status. Therefore, 7500 is the only appropriate response to signify a hijacking situation effectively.