What action should be taken if a diver omits a decompression stop at 40 fsw and a recompression chamber is unavailable?

Returning the diver to the depth of the first decompression stop is the appropriate action in this scenario because this method can help mitigate the risk of decompression sickness. When a diver ascends too quickly and omits a required decompression stop, nitrogen bubbles can form in their tissues and bloodstream, potentially leading to serious health issues. By returning the diver to a shallower depth, such as the depth of the first decompression stop, the surrounding pressure can help reduce the size of any nitrogen bubbles that have formed, allowing them to safely dissipate.

This approach is a safety measure that allows for the gradual elimination of nitrogen from the body without the immediate risks associated with surfacing unrestricted. It provides a more controlled environment for the diver and minimizes the chance of developing symptoms related to decompression sickness, which can be life-threatening.

In contrast, other options such as sending the diver to another dive station or allowing unrestricted ascent could increase the risk of severe medical complications. Thus, safely managing the condition by returning to the appropriate depth is crucial until further treatment can be arranged.