Final answer:
The introduction 'We the People...' in the U.S. Constitution's Preamble explains the reasons for writing the Constitution, to form a government that would ensure justice, tranquility, defense, welfare, and liberty for all citizens.
Step-by-step explanation:
The introduction to the U.S. Constitution, beginning with the words 'We the People of the United States, in order to form a more perfect Union...' serves a foundational purpose.
This preamble outlines the reasons for the writing of the Constitution. Its key functions include establishing a just government system, ensuring peace within the nation, providing defense, promoting wellbeing, and securing liberty for the present and future generations. Consequently, the correct answer is A. To explain the reasons for the writing of the Constitution. It is not specifically about the Bill of Rights, the amendment process, nor the Declaration of Independence; those elements address other facets of the nation's founding documents and history.