"Federalism" means that there are many levels of government--like in the US! We have the national, state, county, and city governments, among others. So what shows this hierarchy? The oath of the president only shows the national government. (A is wrong.) The amendment process is carried out at the state level. (B is wrong.) The branches are at the national level. (C is incorrect.)
The Supremacy Clause states that if national and state (or any other level of government) laws are in conflict, the national law overrules it. This applies to ALL levels of government!
Answer: D. Supremacy clause
