Which of the following is a primary source of sulfur dioxide emissions?

Coal burning is a primary source of sulfur dioxide emissions because coal is often high in sulfur content. When coal is combusted for energy, the sulfur within it is released into the atmosphere as sulfur dioxide. This gas can contribute to air pollution and is a significant factor in the formation of acid rain, which has detrimental effects on the environment.

Automobile exhaust primarily involves the combustion of gasoline or diesel, which does not typically contain significant amounts of sulfur compared to coal. Natural gas combustion emits relatively low levels of sulfur dioxide since natural gas has much less sulfur content. While the decay of vegetation does release a variety of gases, it is not a major source of sulfur dioxide compared to the combustion of fossil fuels like coal. Therefore, the combustion of coal stands out as the principal contributor to sulfur dioxide emissions among the given options.