The correct answer is option 1: Smog. Smog is a thick fog-like layer in the atmosphere that is formed when smoke and fog combine. It is typically caused by the burning of fossil fuels in vehicles, industrial processes, and power plants, which release pollutants such as nitrogen oxides and volatile organic compounds. These pollutants react with sunlight to create a photochemical smog, which is commonly seen in urban areas with high levels of air pollution. Smog can have harmful effects on human health, leading to respiratory problems and eye irritation. It can also negatively impact the environment, causing damage to plants and reducing visibility. Thus, smog is a result of the combination of smoke and fog, creating a distinctive fog-like layer in the atmosphere.