Which of the following gases is not considered combustible?

Carbon dioxide is not considered combustible because it is a product of combustion and does not support the combustion process itself. In a combustion reaction, a fuel reacts with oxygen to produce energy, heat, and light. While gases like methane, hydrogen sulfide, and ethane are flammable and can produce combustion when ignited, carbon dioxide does not have the capability to burn or release energy through combustion.

In fact, carbon dioxide is often used as a fire extinguishing agent due to its ability to displace oxygen, which is necessary for fires to continue burning. This property underscores its non-combustible nature. Thus, in the context of the question regarding which gas is not combustible, carbon dioxide stands out as the correct answer.