We hold these truths to be self-evident, that all men are created equal, that they are endowed by their creator with certain unalienable rights, that among these are life, liberty, and the pursuit of happiness.
(2) that to secure these rights, governments are instituted among men, deriving their just powers from the consent of the governed.
(3) that whenever any form of government becomes destructive of these ends, it is the right of the people to alter or abolish it, and to institute a new government….
—the declaration of independence, 1776
what concept is best described by the third sentence?

The third sentence is a clear example of the concept of social contract.

Explanation:

The idea of social contract became important during the Enlightenment because it established that the power lies in the people and not in the figurehead of government.  The French writer Rousseau is one of the Enlightenment thinkers that popularized the term.  This concept goes on to explain that if the government is not serving its people and is not acting in the people's best interest, then the people have the right to remove it and place a new government in its place.  This can clearly be seen in the third sentence.

there are two main types of historical sources: primary sources and secondary sources. a primary source is something that originates from the past. it can be a chronicle, a piece of pottery, or even a piece of glacial ice that gives us climate data about the levels of atmospheric carbon one thousand years ago.

i am pretty sure it is a