**Introduction**

Smog is a type of air pollution that is characterized by a combination of smoke and fog. It is formed when smoke or other pollutants combine with fog or water vapor in the air. The correct answer to the question is option 'C' - smoke and fog. This answer can be explained in detail as follows:

**Explanation**

1. **Smoke**: Smoke is a mixture of small particles, gases, and other combustion products that are released into the air when something is burned. It is produced by various sources such as industrial processes, vehicles, and wildfires. Smoke contains pollutants such as carbon monoxide, nitrogen oxides, sulfur dioxide, and particulate matter.

2. **Fog**: Fog is a cloud-like mass of water droplets that is suspended in the air near the ground. It occurs when the air cools and reaches its dew point, causing water vapor to condense into tiny droplets. Fog reduces visibility and can create hazardous driving conditions.

3. **Combination**: When smoke and fog combine, they form smog. The smoke particles and pollutants in the air mix with the water droplets in the fog, creating a thick, hazy, and polluted atmosphere. The pollutants in the smoke can react with sunlight and other atmospheric chemicals, leading to the formation of secondary pollutants such as ozone and fine particulate matter.

4. **Formation**: Smog is typically more prevalent in urban areas where there are high levels of pollution from vehicles, industries, and other sources. The emissions from these sources mix with the moisture in the air, resulting in the formation of smog. Weather conditions such as calm winds and temperature inversions can also contribute to the build-up of smog.

5. **Effects**: Smog has several negative effects on both human health and the environment. It can cause respiratory problems, eye irritation, and worsen existing lung conditions. Smog can also harm plants, crops, and ecosystems. The pollutants in smog can contribute to the formation of acid rain and contribute to global warming.

**Conclusion**

In conclusion, smog is formed by the combination of smoke and fog. Smoke, which contains pollutants, mixes with the water droplets in fog to create a polluted and hazy atmosphere. Smog has harmful effects on human health and the environment, making it a significant concern for air quality in many urban areas.

Smog is a kind of air pollution, originally named for the mixture of smoke and fog in the air. Classic smog results from large amounts of coal burning in an area and is caused by a mixture of smoke and sulfur dioxide..