What action is being taken when N1 and N2 receive NITS during a flight?

The action taken when N1 and N2 receive NITS during a flight refers specifically to "Notification in Tactical Situations," which typically involves coordination for cabin operations. This process is crucial for ensuring effective communication and organization among crew members, particularly during routine operations or any situation that may require collaborative action between the cockpit and the cabin crew. This coordination facilitates coordinated responses to any developments that may arise during the flight, promoting safety and efficiency.

In this context, other options do not align with the specific nature of NITS. A checklist process would pertain more to pre-determined procedural steps rather than real-time operational coordination. A safety briefing procedure is usually related to the initial safety protocols discussed with passengers before takeoff. An emergency situation response is typically reserved for situations that are immediately life-threatening or require urgent action, rather than the standard operational communications highlighted by the NITS protocol. Therefore, the appropriate action linked to NITS is indeed a coordination for cabin operations.