The correct answer to this open question is the following.
What the Declaration of Independence say about who has power in a nation? is "The People."
The Declaration of Independence is one of the most important documents in the history of the United States. It was drafted by a Committee of Five prominent politicians led by Thomas Jefferson. The other four were Robert Livingston, Benjamin Franklin, John Adams, and Roger Sherman. The Declaration was promulged on July 4, 1776, and it clearly expresses that the power to govern relies on "The People."