Final answer:
The clause that shows the national law supersedes state law when there is a conflict is the Supremacy Clause, contained in Article VI of the U.S. Constitution. It asserts that federal laws and treaties are the supreme law of the land and override conflicting state laws.
Step-by-step explanation:
When a national law and a state law conflict, the clause in the Constitution that illustrates the national law prevailing is the Supremacy Clause. This clause is found in Article VI and establishes that the Constitution, federal laws, and treaties are the supreme law of the land. It mandates that judges in every state must follow the Constitution, even when there is a contradiction with state laws. The Supremacy Clause clearly supports federal authority and the idea that national laws take precedence over state laws when there is a dispute between the two levels of government.
In addition, the source of powers "reserved" to the states is the Tenth Amendment, which delineates the division of power between the federal government and the states. The Full Faith and Credit Clause, which is mentioned but is not the answer to the primary question, concerns interstate relations and ensures that states respect each other's laws and judicial decisions, not the supremacy of federal over state law.