Answer:
A. The Constitution
Explanation:
From all the choices, the U.S. Constitution is the only one that outlines the structure of the federal government. Article I, II and III establish the structure, main duties, and powers of each of the three branches of the government (Legislative, Executive and Judicial) and establishes the principles under they should they act: the principle of separation of powers and the principle of checks and balance.
As for the remaining articles, Article 4 deals with interstates relationships and the responsibilities that the federal government has to each States, Article 5 explains how the Constitution can be changed, Article 6 talks about debts, and establishes the constitution, federal laws, and treaties as the supreme law of the land, and lastly, Article 7 explains how many states need to ratify the Constitution.
