ANSWER:
The right answer is option a, managing a trust made up of Standard Oil and allied companies.
Step-by-step explanation:
- John D. Rockefeller had a monopoly in the oil industry. He managed a trust made up of "Standard Oil and allied companies".
- All the demands of oil were fulfilled by the company of John D. Rockefeller because he used to sell the oil in lesser cost than the production cost in his early stages.
- Once he had the monopoly in the market then he started earning the immense amount of money.