The correct answer is option 'D', the Constitution. The functions and responsibilities of the Supreme Court are defined by the Constitution of the United States.

Explanation:
1. The Constitution of the United States: The Constitution is the supreme law of the land in the United States. It was adopted in 1787 and has been amended multiple times since then. The Constitution establishes the framework of the federal government and outlines the powers and responsibilities of its three branches: the legislative, executive, and judicial branches.

2. Separation of Powers: The Constitution establishes a system of government based on the principle of separation of powers. The judicial branch, which includes the Supreme Court, is responsible for interpreting and applying the law.

3. Article III: Article III of the Constitution specifically establishes the judicial branch. It outlines the authority and jurisdiction of the federal courts, including the Supreme Court. It grants the Supreme Court the power to decide cases involving federal law, constitutional issues, and disputes between states.

4. Powers of the Supreme Court: The Constitution grants the Supreme Court both original and appellate jurisdiction. Original jurisdiction means that the Supreme Court has the authority to hear certain types of cases directly, without them going through lower courts first. Appellate jurisdiction means that the Supreme Court can review decisions made by lower courts.

5. Checks and Balances: The Constitution also establishes a system of checks and balances among the three branches of government. This means that each branch has some oversight over the others to prevent any one branch from becoming too powerful. The Supreme Court plays a crucial role in this system by ensuring that laws and actions of the other branches are in accordance with the Constitution.

In conclusion, the Constitution of the United States defines the functions and responsibilities of the Supreme Court. It establishes the authority and jurisdiction of the Court, outlines its powers, and ensures the separation of powers among the three branches of government.