Which is the best definition of federalism?
-federalism enables the states and the national government to share power.
-federalism creates the separation of powers between the three branches of government.
-federalism encourages the practice of direct democracy at the state level rather than the national level.
-federalism provides for the regular election of state and national officials.
-federalism gives congress powers that were not originally enumerated in the constitution.

Federalism enables the states and the national government to share power.

The term "Federalism" means the power is divided between the national government and other governmental units, so that narrows it down to both A and B. However the question doesn't ask what specifically was adopted by the US in that. Federalism has been adopted by several countries such as Canada, Australia, Brazil, China, India, and more however all of those don't have the same three branches but share power based on states or provinces. Also I took the test and this is the right answer.

its many islands, high mountains, and remote peninsulas isolated population centers.