At first I thought it was putting a comma between Please & mom or switching those two words....
The correct answer is the C. Constitution for the United States of America.
Explanation
A preamble is a part of the introductory part of a law in which the reasons for which it originated are contextualized and exposed. According to the foregoing, the preamble of the United States allows us to know the context in which the Constitution originated and its purpose as a normative framework of a nascent nation. So the correct answer is the "C. Constitution for the United States of America" because fragment clearly establishes the purpose of this document.
I would agree with you - the conclusion should leave readers with an important thought at the end.
This thought should never be new, or opposing to the previous ideas you had been writing before. It also shouldn't be confusing, but quite conversely - it should explain everything and put everything in place.