What founding document was written in 1787?

What does this answer mean?

The U.S. Constitution was written in 1787. USCIS accepts "the Constitution" or "the U.S. Constitution."

That year, delegates from the states met in Philadelphia, at the Pennsylvania State House. Congress had called the meeting to revise the Articles of Confederation, the country's first constitution, which had left the national government weak. The delegates, who chose George Washington to lead the convention, wrote a new plan instead. They signed the Constitution on September 17, 1787.

The Constitution then had to be approved by the states, and it said that approval by nine states would be enough for it to take effect. During that debate, the Federalist Papers argued for approving it. George Washington became the first president under the Constitution in 1789.

Don't confuse it with the Declaration of Independence, which was written in 1776.

At the interview, say "the Constitution."