During the Progressive Era (early 20th century), muckrakers played a significant role in American society by:

The correct answer to the question "During the Progressive Era (early 20th century), muckrakers played a significant role in American society by:" is Option B: Exposing corruption and social injustices..

The other options — A) Promoting isolationist foreign policies., C) Advocating for lower taxes on corporations., D) Supporting unlimited immigration. — are incorrect for this GED Exam Prep question. As explained above: Muckrakers were investigative journalists who exposed corruption in politics and business, as well as social injustices, prompting public outcry and contributing to the Progressive reform movements.