Answer:
The United States established the Monroe Doctrine because:
D. The U.S. was concerned that Europeans would regain colonies in Latin America and hurt trade.
Step-by-step explanation:
The Monroe Doctrine, which takes its name after President James Monroe, the 5th president of the United States, was established to oppose European colonialism. In 1823, the United States had already been a free country for a few decades. However, independence processes were still happening all over Latin America. Spanish and Portuguese colonies were only then declaring their freedom.
Monroe's doctrine had the purpose of preventing countries such as Spain and Portugal from taking over their colonies again. It basically stated that the U.S. would not allow European interference in America. Of course, the reasons behind such a decision were not completely altruistic. It was much more profitable for the U.S. to engage in trade with free countries. Colonies are not allowed to engage in trade with any other country besides the one colonizing them.