The correct answer is C. Helium.
Helium is the most abundant element in the universe, but it is only a very small fraction of the air we breathe. It is a colorless, odorless, and tasteless gas that is lighter than air. Helium is used in deep sea diving because it is less soluble in blood than nitrogen. This means that divers can breathe helium-oxygen mixtures without the risk of nitrogen narcosis, a condition that can cause confusion, impaired judgment, and even death.
Argon, xenon, and krypton are also inert gases, but they are not as effective as helium for deep sea diving. Argon is the third most abundant element in the Earth’s atmosphere, but it is more soluble in blood than helium. Xenon and krypton are even more soluble in blood than argon. This means that divers who breathe mixtures of these gases are at a greater risk of developing nitrogen narcosis.
In conclusion, helium is the best gas to use for deep sea diving because it is less soluble in blood than nitrogen. This means that divers can breathe helium-oxygen mixtures without the risk of nitrogen narcosis.