Detailed Explanation:

Answer: Option 4 — Power plants using fossil fuels

Power plants burning fossil fuels, particularly coal, are the largest source of sulfur dioxide (SO₂) emissions globally according to the EPA. Coal contains high levels of sulfur, and when combusted for electricity generation, it releases substantial amounts of SO₂ into the atmosphere. While ships using bunker fuel, metal extraction processes, and locomotives also contribute to SO₂ emissions, their combined contribution is significantly lower than emissions from fossil fuel-based power generation.

📝 Short Notes: Sulfur Dioxide (SO₂) Emissions

• Primary Source: Coal-fired power plants are responsible for approximately 65-70% of global SO₂ emissions, as coal contains 0.5-3% sulfur by weight.
• Ships: Maritime vessels using heavy bunker fuel (high sulfur content) are the second major source, especially in coastal and port areas; regulations like MARPOL Annex VI now limit sulfur content in marine fuels.
• Metal Smelting: Extraction of metals like copper, zinc, and lead from sulfide ores releases SO₂ during roasting and smelting processes, contributing 5-10% of total emissions.
• Locomotives: Diesel-powered trains contribute minimally to SO₂ emissions due to lower sulfur content in diesel fuel and relatively smaller fuel consumption compared to power plants.
• Environmental Impact: SO₂ causes acid rain, respiratory problems, and contributes to particulate matter (PM2.5) formation; it also damages vegetation and corrodes buildings.
• Control Measures: Flue gas desulfurization (FGD/scrubbers), switching to low-sulfur fuels, and renewable energy transition are key mitigation strategies.