Final answer:
The U.S. Supreme Court gains the authority to invalidate an act of Congress as unconstitutional through the power of judicial review, which was established in the case of Marbury v. Madison in 1803.
Step-by-step explanation:
The U.S. Supreme Court's authority to invalidate an act of Congress as unconstitutional comes from the power of judicial review. This power was established by the Court in the case of Marbury v. Madison in 1803. The Court ruled that it had the power to determine the constitutionality of laws passed by Congress, and could declare them invalid if they violated the Constitution.
Learn more about Judicial review