What action should be taken if cabin pressure CAS warning lights up?

When the cabin pressure warning light illuminates, indicating a potential loss of cabin pressure, the correct response is to don oxygen masks. This is important because a loss of cabin pressure can lead to hypoxia, which is a deficiency in the amount of oxygen reaching the tissues, making it vital for pilots and passengers to have access to supplemental oxygen. Wearing oxygen masks ensures that everyone on board can maintain adequate oxygen levels until the situation is stabilized.

Addressing hypoxia is critical, and immediate action helps prevent any adverse effects on cognitive function or physical abilities. While the other options could be relevant in different contexts—such as notifying ATC about the situation, checking the cabin door to ensure it is securely closed, or considering altitude adjustments—donning oxygen masks directly addresses the immediate safety concern posed by the cabin pressure alert.