What should aircrew do immediately after take-off in the event of a hijacking?

Setting the transponder to 7500 is the immediate action aircrew should take in the event of a hijacking. This specific squawk code is universally recognized as a signal indicating that an aircraft is under unlawful control. By selecting this transponder code, the aircraft communicates to air traffic control that it is experiencing a hijacking situation without alerting the hijackers to the crew's actions. This discreet method allows air traffic controllers to be aware of the situation and respond appropriately, coordinating with law enforcement and emergency services.

The other options, while important in the context of overall emergency response, do not prioritize the initial signaling of a hijacking situation immediately after take-off. Alerting law enforcement authorities can take place but typically occurs after the transponder has been set, as they need the immediate alert from air traffic control first. Activating emergency landing procedures and communicating with the control tower are also critical actions but may come after setting the transponder if the situation demands more immediate attention to signaling the hijacking.