The purpose of the constitution is presented in the
a. articles
b. bill of rights.
c. amendments.
d. preamble.

The purpose of the Constitution is presented in the "Preamble" since this is the first thing that one would read when reading the Constitution, and the writers of the Constitution wanted to make sure that their intent was clear. 

Primary sources are usually  more accurate and are written by someone who experienced it or witnessed it. secondary sources are those who heard of it after the fact that can lose details and or twist the event in a light that benefits them.