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

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. 

Assemblies do not have written codes of laws. laws are matters for magistrates and courts, not assemblies. codes of law are things such as civil law, criminal law, and constitutional law. what assemblies have are procedures regarding debating, voting and other business they might carry out.  also note that rome had three assemblies: the assembly of the soldiers, the assembly of the tribes, and the plebeian councils.