(a). Locomotives using fossil fuels
(b). Ships using fossil fuels
(c). Extraction of metals from ores
(d). Power plants using fossil fuels
Explanation:
According to the Environmental Protection Agency (EPA), the largest source of sulfur dioxide (SO2) emissions comes from the burning of fossil fuels by power plants and other industrial facilities [EPA: Sulfur Dioxide Basics]. While locomotives, ships, and metal extraction also contribute to SO2 emissions, they are considered smaller sources compared to power plants.
The correct statement is (d) : Power plants using fossil fuels