What state is achieved when the pressure of nitrogen is balanced between the air and a diver's body?

The correct choice is equilibrium, which describes a state in which the partial pressure of nitrogen in the diver's body is equal to the partial pressure of nitrogen in the surrounding water and air. In diving, this balance is crucial to prevent nitrogen from dissolving into the body's tissues.

When a diver descends, the increased pressure underwater causes nitrogen from the breathing gas to be absorbed into the tissues. Equilibrium is achieved when the amount absorbed matches what is present in the surrounding environment, meaning no net movement of nitrogen occurs into or out of the body. This state helps prevent issues such as decompression sickness, which can occur if a diver ascends too quickly before equilibrium is reached.

In contrast, hypoxia refers to a state where there is insufficient oxygen in the body, which is not directly related to the pressure of nitrogen. Decompression involves the controlled ascent to allow the body to eliminate excess nitrogen safely, and hyperventilation is an increased breathing rate that can lead to a range of physiological issues but does not pertain directly to the balance of nitrogen pressure.