Final answer:
During the times of Rockefeller and Carnegie, the United States would have been closest to a Free Market system due to minimal government regulation.
Step-by-step explanation:
The economic system in the United States during the times of Rockefeller and Carnegie, a period with little regulation, would have been closest to a Free Market system. This type of economy has minimal government intervention, which allows for private businesses to operate in competition with one another. While the U.S. economy has since incorporated more regulatory measures, leading to a more mixed economy, most economies worldwide combine elements of market, command, and traditional systems.