The correct answer is option 4, Helium. Deep sea divers use a mixture of gases called breathing gas to supply oxygen and prevent the harmful effects of the increased pressure underwater. One of the main components of breathing gas is an inert gas which replaces the nitrogen in normal air.

Option 1: Krypton is not commonly used in deep sea diving as it is expensive and has potential health hazards.

Option 2: Neon is also not commonly used in deep sea diving due to its limited availability and high cost.

Option 3: Argon is not typically used for deep sea diving as it can cause narcosis at high pressures.

Helium, on the other hand, is the most commonly used inert gas in deep sea diving. It has several advantages, such as being non-toxic and non-reactive, and it does not cause narcosis at high pressures. It also has low density, allowing divers to breathe easily and reduce the risk of decompression sickness. Therefore, option 4, Helium, is the correct answer.