Which of the following gases is a product of incomplete combustion?

Incomplete combustion occurs when there is insufficient oxygen available to fully oxidize the fuel being burned. This typically results in a mixture of products, including various hydrocarbons and gases that indicate inefficiency in the combustion process.

Carbon monoxide is a result of incomplete combustion because it forms when carbon-containing fuels do not have enough oxygen to convert all the carbon into carbon dioxide. Instead, some of the carbon is released as carbon monoxide, a colorless and toxic gas, which can pose serious health risks.

In contrast, carbon dioxide is a product of complete combustion, signifying that all the carbon in the fuel has been fully oxidized. Hydrogen and oxygen are not direct products of combustion in the same way. Hydrogen may appear in some reactions, but typically it is not a byproduct of combusting carbon-based fuels. Oxygen is a reactant in combustion rather than a product and would not be formed as a result of burning fuel.

Thus, carbon monoxide is the appropriate choice as it directly represents a product of incomplete combustion, highlighting the presence of unburned carbon from the fuel.