Final answer:
The correct answer is option c. Nitrogen oxide is the least common indoor air pollutant among the other options listed, as it originates mostly from outdoor sources like vehicle emissions and industrial activities.
Step-by-step explanation:
Among radon, asbestos, nitrogen oxide, and lead dust, nitrogen oxide (often referred to as nitrogen dioxide, NO2, or one of the various nitrogen oxides, NOx) is the least common as an indoor air pollutant.
While radon is a naturally-occurring radioactive gas that can seep into buildings and is linked to lung cancer, asbestos used in building materials is a serious hazard causing lung diseases, and lead dust from various sources can be harmful if inhaled or ingested, nitrogen oxide primarily originates from outdoor sources such as vehicle emissions and industrial processes. It is not commonly found in indoor environments unless there is a significant infiltration from outdoor air or an unusual indoor combustion process taking place.