Final answer:
Jeanette Rankin was the first woman to serve in Congress, where she was a committed pacifist and played a key role in advancing women's suffrage.
Step-by-step explanation:
The first woman to serve in Congress was C) Jeanette Rankin. As a field worker for the National American Women's Suffrage Association, Rankin played a significant role in the suffrage movement, contributing to victories in North Dakota and Montana. After her efforts, she made history as the first female member of Congress, using her position to advocate for peace and voting against the United States entering World War I, consistent with her steadfast pacifist beliefs. In addition to her anti-war stance, Rankin was also instrumental in the fight for women's suffrage on a national level.