Which of the following could lead to high carbon monoxide (CO) emissions in a vehicle?

A rich fuel mixture can lead to high carbon monoxide (CO) emissions in a vehicle because when there is an excess of fuel compared to the amount of air in the combustion chamber, the engine cannot burn all of the fuel efficiently. This incomplete combustion results in higher CO production, as carbon monoxide is a byproduct of burning hydrocarbons when oxygen is limited. In this scenario, the vehicle is not able to attain optimal air-fuel ratios needed for effective combustion, which directly contributes to increased CO emissions.

The other factors, while they may affect vehicle performance in different ways, do not have a direct impact on carbon monoxide levels like a rich fuel mixture does. Low oil levels primarily affect engine lubrication and could lead to engine wear or failure rather than emissions. A faulty air conditioning system typically influences cabin temperature and comfort, not the combustion process. Also, dirty windshield wipers are related to visibility and safety rather than emissions, as they do not influence the combustion of fuel. Therefore, among the given options, a rich fuel mixture stands out as the primary cause of elevated CO emissions.