Answer:
A. Constitution, democratic republic
Step-by-step explanation:
The U.S. Constitution was passed in 1776, and it is the oldest Constitution in the world. This also makes the United States the oldest democratic republic in the world, because that is the system that the Constitution established for the country, and that system has been held ever since.
The idea of a democratic, representative republic was influenced by Ancient Athenian Democracy, the Roman Republic, and the ideas of the Enlightenment. The Framers of the Constitution took inspiration from all these ideas and devised the Constitution according to them.