Which of the following are not major sources of sulfur dioxide pollution?

Electrical substations are indeed not major sources of sulfur dioxide (SO2) pollution. Sulfur dioxide primarily originates from activities that involve the combustion of fossil fuels, particularly coal and oil, which release SO2 as a byproduct. Metal smelters and refineries are also significant contributors to SO2 emissions due to their processing of metal ores and fossil fuels, which can contain sulfur.

In contrast, electrical substations are facilities that transform voltage levels for electricity distribution. While they play a crucial role in the electrical grid, they do not directly produce sulfur dioxide as they do not involve processes that burn fossil fuels or generate emissions like the other listed options do. Therefore, they are accurately categorized as not being a major source of sulfur dioxide pollution.