Among the options provided, carbon monoxide is the pollutant most commonly associated with poor air quality. It is a colorless, odorless gas produced by incomplete combustion of fossil fuels and can be harmful to human health when present in elevated levels.
While nitrogen can refer to nitrogen oxides (NOx) which are also pollutants linked to air quality issues, the specific mention of "nitrogen" in your options is too broad. Oxygen and water vapor are not considered pollutants in the same context. Therefore, carbon monoxide is the best choice here.