Final answer:
The Revenue Act of 1913, also known as the Underwood Tariff Act, passed in 1913, was the first significant reduction in tariff rates since the Civil War, a key part of President Wilson's New Freedom agenda.
Step-by-step explanation:
The act passed in 1913 that represented the first substantial reduction of tariff rates since the Civil War was the Revenue Act of 1913, otherwise known as the Underwood Tariff Act. When President Wilson took office in March 1913, he aimed to implement his New Freedom agenda, which included the reduction of tariffs to foster international trade and competition. To accomplish this, Wilson convened a special session of Congress in April 1913 to address the tariff, resulting in the Revenue Act of 1913, which lowered tariffs by approximately 15 percent and eliminated them entirely on some imports. This was a significant shift from previous policies and the first major reduction since the high tariff rates of the Civil War.