Final answer:
The U.S. Constitution is the supreme law of the land, which is established by the Supremacy Clause within Article VI of the Constitution.
Step-by-step explanation:
The document that is considered the supreme law of the land is the U.S. Constitution. According to Article VI of the U.S. Constitution, the Constitution itself, along with the laws and treaties made under its authority, supersedes any other laws, including those of the individual states. It contains the Supremacy Clause, which unequivocally establishes it as the highest authority. The Declaration of Independence, while a critical document, was the product of the Second Continental Congress and declared the thirteen colonies free from British rule. It did not create laws or governance structures like the Constitution did. The Magna Carta and Supreme Court opinions, although significant in their own rights, do not serve as the supreme law of the land.