When was the Constitution of the United States ratified?

The Constitution of the United States was ratified in 1787. This year is significant because it marks the completion of the document during the Constitutional Convention held in Philadelphia. After weeks of debate and discussion among the delegates, the Constitution was signed on September 17, 1787. Subsequently, it went through the process of ratification by the states, with the required nine states approving it by June 21, 1788, allowing the document to take effect.

The other years mentioned do not align with the ratification of the Constitution. 1776 is known for the Declaration of Independence, which established the colonies' intent to gain independence from British rule. 1789 is the year when the Constitution went into effect after the ratification process was completed, and the first Congress met under it. 1791 marks the year when the Bill of Rights, the first ten amendments to the Constitution, was ratified. Thus, 1787 is the correct answer as it is the year when the Constitution itself was ratified.