What is the initial action if "N1 to the flight deck" is heard during boarding?

The correct action in response to "N1 to the flight deck" during the boarding process is for N1 and N2 to stop passengers from entering the aircraft. This is crucial because the announcement likely indicates that there is an important communication or safety issue to address in the cockpit, which requires immediate attention without further distraction or delay from boarding activities. Stopping passengers ensures that the flight crew can manage the situation effectively and maintain a safe environment. By halting boarding, N1 and N2 prioritize safety and the operational integrity of the flight, allowing the flight deck crew to focus on resolving the issue at hand.