All but one are the most common gases found in confined spaces

The correct choice identifies chlorine as the least common gas typically encountered in confined spaces compared to the others listed. Confined spaces often present a risk of encountering gases that are a direct result of combustion processes, biological decay, or industrial operations.

Carbon monoxide is a common concern because it is a byproduct of fuel combustion and can accumulate in poorly ventilated areas. Oxygen must also be monitored, as the concentration levels needed to support human life must be maintained; too little oxygen presents a significant safety hazard. Hydrogen sulfide is another hazardous gas commonly found in confined spaces, particularly in industries dealing with sewage, petroleum, or natural gas, and can pose severe health risks even at low concentrations.

While chlorine can be present in certain environments, its occurrence is less frequent when compared to the other three gases listed, making it the outlier in terms of commonality in confined space situations.