How did American women who opposed the war demonstrate their opposition?

They joined the National American Woman Suffrage Association (NAWSA).

They refused to take part in the draft.

They formed the Women's Peace Party.

The answer is: They formed the Women's Peace Party

Explanation:

The American women demonstrated their opposition to World War I by forming the Women's Peace Party (WPP). This organization was established by Jane Addams, an American social worker and activist.

The members of this organization believed that "women were the mother half of humanity" and thus, they needed to do something to stop the war. They called for arm control and its limitation, managing European conflict and striving to remove the causes of war (particularly, economic).

Thus, this explains the answer.