A carcinogenic substance is a substance that has the potential to cause cancer. Among the options given, asbestos dust is a known carcinogen, meaning it has been scientifically proven to cause cancer in humans. Sawdust is not generally considered a carcinogen, although it can cause respiratory problems. Nitrogen (II) oxide and carbon (II) oxide are not typically considered carcinogenic substances, although they can be harmful to human health in other ways. Therefore, the correct answer to this question is "asbestos dust."