Which event is the clearest example of conflict between state and national governments?
a) the civil war
b) the war of 1812
c) the great depression
d) the northwest ordnance

A: The Civil War
Assuming that you're talking about the American (U.S) civil war, that is.

The civil war was a conflict between the rights of the (slave owning) states and the national government. A war about state rights.

The war of 1812 was between 2 independent, sovereign nations.
The great depression was just an economic thing.
The northwest ordnance, I have the least knowledge of, but it still wasn't between states and the national government.
Hope this helps.