The correct answer is C. Everyone is responsible for working together and motivating others to do the same
Step-by-step explanation:
In a text, the main idea is the main point explained by all the details included by the author. In the case of the excerpt presented, Roosevelt focuses on stating the importance for all people in a nation to work on their area and act as leaders by motivating/supporting others to work. This can be seen in "a free nation has the right to expect full cooperation from all the groups" that shows how important it is that everyone works or in "to take the lead in stimulating effort" that shows the responsibility of motivating others. Thus, the sentence that best states the main idea is "Everyone is responsible for working together and motivating others to do the same".