Final answer:
The London smog of December 1952 was mainly caused by the burning of high sulfur coal, leading to a significant amount of sulfur dioxide and other pollutants that contributed to the smog.
Step-by-step explanation:
The fuel most responsible for producing the London smog of December 1952 was high sulfur coal. Industrial smog, also known as London-type smog, is primarily a result of burning coal. The pollution caused by this includes large amounts of soot, fly ash, sulfur dioxide, and other particles. The smog in London was so severe that it left soot on every surface, prompting schoolchildren in cities like Osaka, Japan, to learn of their city as "the City of Smoke." Additionally, the burning of coal was a significant source of CO₂ emissions, with Britain having to significantly revise their coal reserves estimates due to unviable coal seams. The unbearable conditions of the smog led to the Clean Air Act of 1956, which effectively reduced smog formation in the UK.