Final answer:
The Supreme Court's ruling in National League of Cities v. Usery (1976) was significant for establishing the Tenth Amendment as a limit on Congress's powers under the Commerce Clause, which aligns with answer option (A).
Step-by-step explanation:
The Supreme Court's decision in National League of Cities v. Usery (1976) was significant because it established the Tenth Amendment as a limit on the power of Congress under the Commerce Clause, which is the correct answer (A). This case was an example of the Court asserting limits on the scope of the national government's power, particularly in relation to the Commerce Clause's broad interpretation historically used to justify various regulations on local commercial activities. This specific case recognized that while the federal government has both enumerated and implied powers, those areas not exercised by the federal government leave room for the states to wield their powers.
However, this ruling by the Supreme Court would later be overturned by Garcia v. San Antonio Metropolitan Transit Authority (1985), which reasserted broader federal power under the Commerce Clause. Even so, subsequent cases like United States v. Lopez in 1995 began to signal a shift in the Court's approach, imposing limitations on Congress's Commerce Clause powers and thereby indicating a resurgence in concerns for states' rights.