The major activity that enabled Standard Oil to become a large integrated monopoly was refining oil.
Standard oil was able to gain a monopoly in the oil industry at the time by acquiring all of the competing refineries and uniting them underneath the standard oil brand.
This gives them complete control over the United States' oil product market.
Thus, Option B is correct.
What does oil refining entail?
Oil refining is a crucial step in the conversion of crude oil into marketable products like gasoline, lubricants, and kerosene.
Distillation, cracking, coking, reforming, and posttreatment and refining of the products are all components of a conventional oil-refining process.
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