Final answer:
In the event of running out of air underwater, a scuba diver should use their buddy's alternate air source to safely ascend and prevent decompression sickness.
Step-by-step explanation:
If you are a scuba diver and realize you are out of air, the best response is to use your buddy's alternate air source. This is an emergency procedure that should be practiced and understood before diving. Sharing air allows both divers to ascend safely to the surface, ensuring neither diver runs out of air during the ascent. Making a direct ascent to the surface without an air supply is extremely risky and can lead to decompression sickness (DCS) due to the rapid change in pressure. Signaling for help is not an immediate solution, and utilizing your buddy's alternate air source is the quickest way to ensure you have access to air while you both ascend at a safe speed.