What significant action did George Washington take regarding the U.S. Constitution?

George Washington's significant action regarding the U.S. Constitution was to sign it. This act of signing in 1787 marked a crucial endorsement of the document that would lay the foundation for the United States' system of government. Washington, as President of the Constitutional Convention, played a vital role in facilitating the deliberations among the delegates. His support helped to lend credibility and legitimacy to the Constitution, encouraging others to ratify it.

The other options do not accurately reflect Washington's actions regarding the Constitution. He did not draft the Constitution, although he was instrumental in its formation. While he was supportive of the Constitution, he did not oppose it as some contemporaries did. Additionally, the amendment process is a post-ratification aspect of the Constitution itself, which Washington did not directly engage in during its original drafting or signing. His signing symbolized unity and a commitment to the new framework of governance that the Constitution established.