The correct answer is D) bought stock options.
What Rockefeller did to establish the standard oil as a monopoly was to buy stock options from other smaller companies until he became the only owner of most of the oil companies in America.
John D. Rockefeller (1839-1937) founded the Standard Oil Company in 1870. After a decade, Standard Oil controlled 90 percent of the oil in the United States. He bought companies that were competitors or that were part of the oil supply chain and created a monopoly. That is why in 1911, the Supreme Court of the United States ordered the dissolution of the company because it violated antitrust laws.