Which gas is primarily responsible for causing narcosis in scuba diving?

Narcosis in scuba diving is primarily caused by nitrogen. As divers descend into deeper waters, the partial pressure of nitrogen increases, which can lead to a state of euphoria, disorientation, and impaired judgment, commonly referred to as "the bends." This condition occurs because nitrogen is a neutral inert gas that can dissolve in the body tissues and affect the central nervous system, similar to the effects of alcohol.

While oxygen, helium, and carbon dioxide play significant roles in diving physiology, they do not primarily cause narcosis. Elevated levels of oxygen can lead to toxicity at high partial pressures, which is different from narcosis, and helium is often used in deep dives to mitigate the effects of nitrogen. Carbon dioxide can lead to symptoms related to hypercapnia but it is not the gas responsible for the narcotic effects experienced at depth. Thus, nitrogen is the correct choice for identifying the gas that chiefly causes narcosis during scuba diving.