Final answer:
The Ancient Greek and Roman civilizations have most significantly influenced the language, culture, and customs of the United States.
Step-by-step explanation:
The civilizations that most significantly influenced the language, culture, and customs of the United States are B) Ancient Greek and Roman civilizations. The influence of these civilizations can be seen in various aspects of American society. For example, many democratic principles and forms of government in the U.S. are derived from Ancient Greece, and Latin, which was spoken by the Romans, has had an impact on the development of the English language.