Which document outlines the separation of church and state in the United States?

The correct answer is the Constitution, as it includes the Establishment Clause and the Free Exercise Clause within the First Amendment. These clauses collectively establish the principle of separation of church and state by prohibiting the government from establishing a national religion or interfering with individuals' rights to practice their religion freely. This framework has been foundational in American law and supports the idea that government and religious institutions operate independently of one another.

The Bill of Rights, while it does include the First Amendment’s provisions regarding religious freedom, is more of a collection of amendments to the Constitution and does not outline the separation principle independently. The Federalist Papers were a series of essays written to promote the ratification of the Constitution and discuss its principles, but they do not possess the legal authority or direct declaration of separation of church and state. The Declaration of Independence primarily serves as a statement of principles and does not address the relationship between religion and state governance directly. Thus, the Constitution stands as the authoritative document detailing the separation of church and state in the United States.