The correct answer is B. Thomas Jefferson completing the Louisiana Purchase.
President Thomas Jefferson's decision to proceed with the Louisiana Purchase in 1803 is often cited as an example of a president expanding the powers of the office through an action that was not explicitly outlined in the Constitution. Jefferson believed the purchase was advantageous for the United States, but he grappled with whether he had the constitutional authority to acquire new territory. Ultimately, he decided to proceed with the purchase, which significantly expanded the size of the nation and set a precedent for future presidential actions that go beyond explicit constitutional grants of power.