What is one of the immediate actions to take in case of an onboard fire?

Advise the Flightdeck is the correct action to take in the event of an onboard fire. Alerting the Flightdeck is crucial because the flight crew needs to be made aware of the situation immediately in order to take necessary actions, coordinate with air traffic control, and prepare for any potential emergencies. The Flightdeck has the responsibility to assess the situation and assist with the overall management of the incident.

This immediate communication allows the Flightdeck to potentially redirect the aircraft if necessary and informs them of any required emergency protocols. It also helps ensure that all safety measures are followed according to established procedures, which can significantly alter the outcome of the situation.

In contrast to this, evacuating passengers immediately could lead to chaos and confusion without first implementing proper procedures. Ignoring the fire is dangerous, as it puts everyone at risk if the fire worsens. Attempting to extinguish the fire alone without proper equipment or backup could also be perilous, as it may not be effective and could hinder the overall response to the emergency.