Which of the following is not a requirement of the Constitution?

The correct answer, which identifies the option that is not a requirement of the Constitution, is the establishment of a national bank. The Constitution does not explicitly mandate the creation of a national bank. While Congress has the authority to create and regulate a national bank, this action is derived from its powers to regulate commerce and to enact laws that are "necessary and proper" for executing its enumerated powers. The establishment of the national bank is more a matter of legislative decision rather than a constitutional requirement.

In contrast, the other options provided relate directly to essential principles and protections outlined in the Constitution. Guaranteeing freedom of speech is enshrined in the First Amendment, which is a fundamental aspect of American democracy. Defining the structure of government is a key feature of the Constitution itself, detailing the branches of government and their respective powers. Additionally, protecting minority rights is a crucial principle that has been interpreted through various amendments and landmark legislation to ensure that the rights of minority groups are upheld against the majority's will. These elements are intrinsic to the function and philosophy of the Constitution, whereas the establishment of a national bank is not an inherent requirement.