Correct Answer: Idle running

Solution : The correct option is Idle running.

The carbon monoxide (CO) level of an automobile exhaust gas is normally highest under idle running circumstances. The engine of a car is idling while it is running at a low pace or is at rest. The combustion process in the engine may not be as efficient at this period as it is while the vehicle is in motion or under load. Inefficient combustion might result in increased quantities of carbon monoxide in exhaust gases.