Final answer:
The synergistic agent that coupled with asbestos to produce lung cancer is cigarette smoke, which contains numerous carcinogens that enhance the risk.
Step-by-step explanation:
The classic example of synergism in regards to toxic effects involves asbestos coupled with a synergistic agent to produce lung cancer. The synergistic agent in question is D) Cigarette smoke. Tobacco smoke contains numerous chemicals known as carcinogens, which have the ability to cause cancer either by themselves or when combined with other substances. Asbestos exposure on its own is a significant health risk and can lead to diseases such as lung cancer, mesothelioma, and asbestosis. However, when combined with the carcinogens found in cigarette smoke, such as benzopyrene and nitrosamines, the risk of developing lung cancer is significantly heightened.