Final answer:
Refrigeration systems using R-134a should be leak checked with nitrogen due to its inert properties and safety in use. Oxygen poses a fire risk and is not recommended, while carbon dioxide and helium are less standard and practical for such purposes.
Step-by-step explanation:
Systems that use R-134a, which is a type of Hydrofluorocarbon (HFC), should be leak checked with nitrogen. Among the gases listed, nitrogen is the safest and most effective option for pressurizing and testing systems for leaks because it is inert and does not react with the components of the refrigeration system. The use of oxygen is hazardous as it can cause fires or explosions when combined with oils or other combustibles commonly found in refrigeration systems. Carbon dioxide could potentially be used but is not standard practice due to variable pressures and the potential for dry ice formation. Helium could be used due to its small molecule size being great for leak detection, but it is often more expensive and less readily available compared to nitrogen.