I believe is the it is a fact that serves as a supporting detail
It would be a. "We the people of the United States ... do ordain and establish this Constitution for the United States of America."
We can rule out c immediately, because it has dependent clauses standing on its own. B. can also be ruled out, because the clause in-between the hyphen does not read correctly. A. is correct grammatically, but D. is also grammatically correct and it explains it in a quick and understandable way.
7.Flat characters are characters that do not change their roles. They are important because most of the time they play a supporting role to the main character
8.People tend to find the book more interesting when there is a suprise in the book