The Westphalia treaty ended religous wars in Europe by recognizing Europe as a group of equal, independant states,
This is considered one of the most important results of the Thirty Years' War.
First of all, I will explain that popular sovereignty refers to the government based on consent of people. In other words, the source of authority is the people and the government´s power would not be legitimate if it disregard people’s will. Thus, the government is supposed to serve to the people who have free choice.
So, the conventions that reflect the principle of popular sovereignty is the House of Representatives whose members are elected every two years directly by citizens. Plus, the senator are elected by people as well.
I believe the answer here is C
B) It helped to create a common culture in the United States.
