The correct answer is option 2: helium.

Deep-sea divers carry cylinders of oxygen mixed with helium to help them breathe. Helium is a gas that is lighter than air, making it ideal for use in scuba diving. When divers are submerged in water, the pressure increases, which can cause nitrogen in their lungs to dissolve into the bloodstream and potentially lead to decompression sickness or "the bends." To prevent this, helium is mixed with oxygen to dilute the presence of nitrogen and reduce the risk of nitrogen bubbles forming in the body.

Option 1, chlorine, is not used as it is toxic and can cause serious respiratory problems. Option 3, nitrogen, is naturally present in the air we breathe, but it is not used in deep-sea diving due to the risk of decompression sickness. Option 4, ozone, is a gas found in the Earth`s upper atmosphere and is not used for breathing purposes by divers.