Which of the following conditions can lead to the production of carbon monoxide during combustion?

The production of carbon monoxide during combustion is primarily linked to the availability of oxygen. When there is insufficient air supply, the combustion process becomes incomplete. In an ideal combustion environment, a proper balance of fuel and oxygen is necessary for complete combustion—where carbon in the fuel is converted entirely to carbon dioxide. However, when there isn't enough oxygen available, not all of the carbon can oxidize fully. This incomplete combustion results in the formation of carbon monoxide, which is a toxic gas.

In contrast, having excessive oxygen or compressed fuel generally facilitates more complete combustion. High humidity can also affect combustion efficiency, but it does not directly lead to carbon monoxide production as insufficient air supply does. The presence of water vapor may displace some of the oxygen but is not as critical to the chemistry of carbon formation as a lack of air. Thus, insufficient air supply is the condition that directly leads to carbon monoxide production during combustion.