Answer:
a. The principle of self-determination for all nations.
Explanation:
The principle of self-determination for all nations mentioned that a country's approach regarding its sovereignty and foreign policy shall be imposed by the will of the people.
This view is similar with the one that is held by progressives. At that time, progressives advocated for the foreign policies be controlled by the majority of the people, not the government representatives. (this means that if for example, the majority of the people votes to not join the world war I, the government need to obey and stay away from the war to fulfill the will of the people)