Why is the preamble an important part of the constitution?

it declares that the constitution has been democratically ratified

it abolishes the articles of confederation

it establishes the three branches of the government

it affirms the people's right to self-governance

The Preamble an important part of the Constitution because it is the introduction of the Constitution. It states that, "We the people...do ordain and establish this Constitution for the United States of America." 

Based on that, it seems to me as if it's this option: 

 It affirms the people's right to self-governance.