What environmental issue is characterized by polluted air formed from smoke and fog?

Smog is the correct answer because it specifically refers to a type of air pollution that is a mixture of smoke and fog. This phenomenon typically occurs in urban areas where industrial emissions, vehicle exhaust, and other pollutants are released into the atmosphere, particularly under certain meteorological conditions. The presence of moisture in the air can combine with these pollutants to create a thick haze that reduces visibility and can have serious health impacts on populations exposed to it.

Traffic congestion, while a significant urban issue, pertains to the overload of vehicles on roads and does not directly describe a form of air pollution. Sanitation refers to the management of waste and water, focusing on public health issues, and squalor describes conditions of extreme poverty and filth that may arise in living environments but does not specifically relate to air quality. Each of these options addresses different aspects of urban environmental challenges, but smog uniquely captures the essence of air pollution formed by smoke and fog.