Question

Which system did the new government created under the Constitution use? a unitary system with all power concentrated in a central government a presidential system with a unicameral legislature a confederation with a better-organized central government than under the Articles of Confederation a totalitarian system a federal republic

Answer 1

That would be E: a federal republic

Answer 2

The United States chose to use a federal republic model (E) when they were writing the Constitution to frame the construction of the U.S Government. 
As a federal republic power is shared between the central government and federal entities which in the case of the U.S. are states. This created a federal system whereby powers were divided between states and the national government and also a republic whereby citizens would elect representatives to represent their interests.