Final answer:
The U.S. president can change public policy quickly by issuing an executive order, which allows instant policy changes without needing congressional approval.
Step-by-step explanation:
The quickest method by which the U.S. president can change public policy is by issuing an executive order. This tool allows the president to manage operations within the federal government and has the force of law. Executive orders can be enacted without the approval of Congress and can have an immediate effect on public policy, which is vital when the president needs to respond swiftly to events or crises.
While some presidential actions can be challenged by the other branches, notably the legislative and judicial branches, executive orders as a form of direct presidential action generally allow for prompt policy changes. In contrast, urging Congress to pass a law, producing a budget proposal, and posting messages to social media involve more time for debate, approval, or public persuasion and therefore do not offer the same immediacy as an executive order.