Which element is used in deep-sea diving mixtures and as a nonflammable lifting gas in airships?

Helium is the correct choice because it is an inert gas that does not support combustion, making it safe for use in various applications, including deep-sea diving mixtures and as a lifting gas in airships. In diving, helium is often mixed with oxygen to create a breathing gas for deep dives because it reduces the risk of narcosis that can occur with nitrogen at high pressures. For airships, helium provides lift without the flammability risks associated with hydrogen, making it a preferred choice. Its low density and nonreactive nature make it ideal for these specific uses.