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Carbon dioxide is classified as a refrigerant in refrigeration systems due to its thermodynamic properties, which allow it to absorb and release heat efficiently. When used as a refrigerant, it operates on the principles of phase change, meaning it can easily transition between gas and liquid states as it circulates through the system, absorbing heat from the environment in the evaporator and releasing it in the condenser.

In refrigeration applications, carbon dioxide is increasingly popular, especially in commercial refrigeration and heat pump systems, due to its low environmental impact and favorable heat transfer characteristics. It has a lower global warming potential compared to many synthetic refrigerants, making it a more sustainable option in line with current environmental regulations.

Other gases like hydrogen, oxygen, and helium do not serve as effective refrigerants in conventional refrigeration cycles. Hydrogen is flammable and has not been employed as a refrigerant due to safety concerns and its low efficiency in heat transfer. Oxygen, being a reactive gas, poses combustion risks and is not suitable for use in refrigeration cycles. Helium is used in specific cooling applications but lacks the necessary thermodynamic qualities required for typical refrigeration processes. Therefore, carbon dioxide stands out as the appropriate choice among the given options.