Which document serves as the introduction explaining its purpose?

The correct answer is the Preamble, which serves as an introductory statement in the United States Constitution. It outlines the purpose of the document and establishes the goals of the federal government. The phrase "We the People" emphasizes the role of the citizens in the government and sets the tone for the principles that the Constitution aims to uphold, such as justice, domestic tranquility, and the general welfare.

The other options, while significant in their own right, do not serve as introductory documents. The Bill of Rights consists of the first ten amendments to the Constitution that guarantee specific individual liberties and rights. The Federalist Papers are a collection of essays arguing in favor of ratifying the Constitution, providing an analysis of its various components but not serving as an introduction. Amendments refer to changes or additions made to the Constitution after its initial ratification, which do not serve an introductory purpose but rather expand or clarify the original document.