What action should be taken if the air supply runs low?

When air supply runs low during a dive, the primary focus should be on ensuring safety through a controlled ascent. This procedure involves using a buddy's air supply, which is essential in maintaining enough breathable air until you can safely surface. Utilizing a buddy's air not only facilitates a safe ascent but also enables divers to remain calm and manage the situation effectively by providing backup air.

The controlled ascent is essential because ascending too rapidly can lead to decompression sickness, which poses serious health risks. By slowly ascending while sharing air with a dive buddy, both divers can follow safe diving protocols and prioritize their well-being.

In contrast to other options, simply switching tanks with a buddy may not be feasible or safe mid-dive. Signaling the dive master may help in some scenarios, but relying solely on that without taking proactive measures may not be adequate to address the immediate concern of low air. Monitoring the remaining air until the dive ends can lead to dangerous situations, as it does not account for the need to ascend safely when air levels become critical. Thus, actively managing the situation by ascending while using a buddy's air is the most effective and responsible action to take.