The final authority on legal questions in the United States is

The correct answer is the Supreme Court.

The Supreme Court is the most powerful court in the United States. This court, which is considered the judicial branch of the federal government, is responsible for hearing cases in which citizens rights may have been violated. From there, the Supreme Court uses its power of judicial review in order to make a ruling that will effect the entire country.

The concept of judicial review essentially allows the Supreme Court to determine whether a law is constitutional or unconstitutional.