The correct answer is option 4: smoke and fog. Smog is a type of air pollution that occurs when smoke and fog combine. Smoke refers to the particles released into the air from burning substances, such as vehicles and industrial emissions. Fog, on the other hand, is a low-lying cloud that forms near the ground when there is a high level of moisture in the air. When smoke interacts with fog, it creates a thick and hazy mixture known as smog. This mixture contains pollutants such as nitrogen oxides, sulfur oxides, and particulate matter, which can be harmful to human health and the environment. It is important to address the causes of smog and implement measures to reduce its formation to protect air quality.