In an out-of-air emergency, which of the following is considered sharing air?

In the context of an out-of-air emergency, sharing air refers specifically to the act of utilizing another diver's air supply to ensure safety and prevent a potentially life-threatening situation. Using a primary second stage is exactly that; it allows a diver who is low or out of air to access another diver's tank and breathe while ascending to safety. This is a critical skill that is taught during diver training, as it facilitates mutual assistance between divers in distress.

Using a dive computer, swimming to the surface, and using a buoyancy control device do not pertain to the concept of sharing air. A dive computer is a tool for tracking depth, time, and no-decompression limits but does not provide any air supply. Swimming to the surface in an emergency could be necessary, but it would not involve sharing air with another diver. A buoyancy control device assists divers in controlling their buoyancy underwater but is not related to air sharing. Thus, the correct action in an out-of-air scenario is indeed to use another diver's primary second stage.