During a cabin altitude warning, what should be donned by the crew?

In the event of a cabin altitude warning, the correct action for the crew is to don oxygen masks set to 100%. This protocol is crucial because a cabin altitude warning indicates that the cabin pressure has dropped to a level that is unsafe for breathing, which could lead to hypoxia, a condition caused by insufficient oxygen. By using oxygen masks at 100%, crew members are ensured access to a safe oxygen supply, allowing them to maintain cognitive function and perform necessary emergency procedures effectively.

The other options, while important in various emergency situations, are not pertinent to a cabin altitude warning. For example, emergency life vests are typically used in over-water scenarios or egress from the aircraft, survival kits may assist in extended survival situations after evacuation, and ear protection is generally relevant for noise levels during engine operation or in specific circumstances, not directly related to altitude issues. Thus, in the context of a cabin altitude warning, the primary focus is ensuring adequate oxygen supply, making oxygen masks at 100% the appropriate choice.