What is the action required if the cabin differential pressure exceeds the red arc limit?

When the cabin differential pressure exceeds the red arc limit, it indicates a potentially dangerous situation that could lead to structural damage or cabin decompression. The appropriate action in this scenario is to descend immediately. Descending to a lower altitude allows the cabin pressure to equalize with the ambient air pressure outside the aircraft, thus alleviating the stress on the airframe and ensuring the safety of the passengers and crew.

The red arc limit signifies a critical threshold, and exceeding it signals that something is wrong with the pressurization system. Immediate descent is the most effective way to address the issue in order to maintain safety and avoid complications that could arise from continued ascent in a compromised pressurization state.

The other options do not adequately address the urgency of the situation: continuing ascent would exacerbate the problem, declaring an emergency is a necessary procedural step but does not remedy the immediate threat, and resetting the gauge would not actually resolve the underlying issue with cabin pressure. Thus, a prompt descent is the required action to mitigate risks associated with exceeding the red arc limit.