What type of gas is typically produced as a by-product of combustion?

The correct option is carbon monoxide (CO) because it is a common by-product of incomplete combustion of fossil fuels, such as natural gas, propane, and oil. When combustion occurs in a confined space with insufficient oxygen, the carbon atoms in these fuels combine with just a single oxygen atom instead of two, resulting in carbon monoxide rather than carbon dioxide. This makes CO particularly hazardous because it is colorless, odorless, and can be toxic at high levels, leading to poisoning and even fatalities.

In contrast, hydrogen is not typically produced as a by-product of combustion processes involving carbon-based fuels; it usually requires specific conditions or additional chemical reactions to generate. Methane, while a combustible gas and a primary component of natural gas, is not a by-product of combustion but rather the fuel that undergoes combustion. Oxygen is consumed during the combustion process, rather than being produced, as it is required for the reaction to occur.