The correct answer is option 'C': Volcanic eruption.

Explanation:
Volcanic eruptions are natural events that release a large amount of pollutants into the atmosphere. These eruptions emit a mixture of gases, particles, and aerosols that can cause air pollution locally and even impact global climate patterns. Here is a detailed explanation of how volcanic eruptions act as a natural source of air pollution:

1. Introduction:
Volcanic eruptions occur when molten rock or magma rises to the surface of the Earth. This process is accompanied by the release of various gases, ash, and other particulate matter. The emitted substances can have significant effects on the air quality.

2. Emission of Gases:
During a volcanic eruption, several gases are released into the atmosphere. The most common gases emitted include sulfur dioxide (SO2), carbon dioxide (CO2), water vapor (H2O), hydrogen sulfide (H2S), and hydrogen chloride (HCl). These gases can have harmful effects on human health and contribute to the formation of air pollutants.

3. Formation of Volcanic Ash:
Volcanic ash consists of tiny particles of fragmented volcanic rock and glass. These particles are formed when molten magma is explosively ejected from the volcano and rapidly cools in the atmosphere. The ash can travel long distances and remain suspended in the air for extended periods, leading to air pollution.

4. Impact on Air Quality:
The gases and ash released during a volcanic eruption can have several negative effects on air quality. Sulfur dioxide, for example, can react with other compounds in the atmosphere to form sulfuric acid, which contributes to acid rain. Ash particles can cause respiratory problems when inhaled and can also damage machinery, disrupt transportation, and impact agriculture.

5. Global Climate Impact:
In addition to local air pollution, volcanic eruptions can have global climate effects. Large volcanic eruptions can inject massive amounts of sulfur dioxide into the stratosphere, where it combines with water vapor to form sulfuric acid aerosols. These aerosols can reflect sunlight back into space, leading to a cooling effect on the Earth's surface.

6. Examples of Significant Eruptions:
Historically, several volcanic eruptions have caused substantial air pollution. One such example is the eruption of Mount Pinatubo in the Philippines in 1991, which released about 20 million tons of sulfur dioxide into the atmosphere. This eruption caused a temporary cooling effect on the Earth's climate.

In conclusion, volcanic eruptions are natural sources of air pollution. The gases and ash released during these events can have detrimental effects on air quality, human health, and global climate patterns. It is important to monitor and study volcanic activity to better understand and mitigate the impacts of these natural sources of air pollution.