What was the core business that made standard oil a horizontally integrated monopoly?
refining oil
transporting oil to customers
building oil pipelines
finding new uses for oil

B) refining oil

just took test :)

Standard Oil started its business in 1870 by John D. Rockefeller and Henry Flagler as a corporation in Ohio. It became the largest oil refinery of its time and participated in every step of the oil process, including producing, refining,  transporting, and marketing company. To answer this question lets understand what horizontal integration means: When a company increase the production of a good via expansion or acquiring the competitors at the same level of the value chain. In this matter, Standard Oil horizontally integrated its monopoly with the refining oil, buying competitors or using aggressive pricing to destroy them.