Answer: A. It describes the relationship between federal and state power.
The Supremacy Clause is considered to be the root of federalism because it describes the relationship between federal and state power. Located in Article VI, Paragraph 2 of the United States Constitution, the Supremacy Clause establishes that all federal laws of the US are supreme laws and should therefore be bound to by all states and federal judges and officers. With this, federal law will stand in the case of a conflict existing between a state and a federal law.
The federal law will always prevail. The power of the federal law cannot be overemphasized. It explains the supreme power of the federal government by amplifying the power it has above all individual states. Federal laws take preference over state laws. This way it describes the relationship between federal power and state power.