Which of the following should not be used as a test medium for testing gas piping?

Using oxygen as a test medium for testing gas piping is not advisable due to the inherent risks associated with its use. Oxygen supports combustion, meaning that in the presence of a flammable gas or material, it can create a highly explosive environment. This characteristic makes oxygen a dangerous choice for testing, as any leaks or fumes could lead to a fire or explosion in the presence of combustible materials.

On the other hand, hydrogen, nitrogen, and carbon dioxide are safer alternatives because they do not support combustion. Nitrogen, for instance, is an inert gas that is often used in pressure testing pipelines because it is non-toxic and does not react with other substances under normal conditions. Carbon dioxide, while generally safe, can pose health hazards in confined spaces but is not flammable, making it less hazardous than oxygen for such testing. Hydrogen is flammable but is typically used in controlled environments where the risk is managed. Thus, oxygen is the clear outlier when selecting a safe test medium for gas piping.