The correct answer is D. 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 developing nitrogen narcosis, a condition that can cause confusion, impaired judgment, and even death.
Xenon, krypton, and argon are also inert gases, but they are not used in deep sea diving because they are more soluble in blood than helium. This means that divers who breathe these gases would be more likely to develop nitrogen narcosis.
In addition, xenon, krypton, and argon are more expensive than helium. This makes them less practical for use in deep sea diving.