Final answer:
John D. Rockefeller held a monopoly in the oil industry by organizing smaller oil companies into a trust and dominating the market.
Step-by-step explanation:
John D. Rockefeller held a monopoly in the oil industry.
Rockefeller was the founder of Standard Oil Company, which became a conglomerate that controlled many smaller oil companies. By using trusts, he was able to dominate the market and raise oil prices once he controlled it.
His business practices were often perceived as predatory and aggressive, leading to criticism from progressive reformers and other critics.