Final answer:
Rockefeller built his monopoly through horizontal integration, which involved merging with other oil companies or driving competitors out of the market. This allowed him to control all aspects of the oil refining industry and eliminate competition.
Step-by-step explanation:
Rockefeller's monopoly was built through horizontal integration, which involved merging with other oil companies or driving competitors out of the market. Horizontal integration allowed Rockefeller to control all aspects of the oil refining industry, from acquisition of companies involved in raw materials to the delivery of the final product. By controlling a large portion of the market, Rockefeller was able to lower prices and eliminate competition, leading to the establishment of his monopoly.