Final answer:
Robert La Follette, not listed in the provided options, was a leading progressive who served in the US Senate. Frederick Douglass was an abolitionist but did not serve in the Senate.
Step-by-step explanation:
The correct answer to the question of which individual was a leading progressive who served in the US Senate is not among the options provided. While Frederick Douglass was a leading voice in the abolitionist movement and a significant figure in the early nineteenth century, he did not serve in the US Senate. The information provided helps us to identify Robert La Follette as a three-term senator and a leader of the Progressive wing of the Republican Party during the progressive era. He was an abolitionist, author, and orator who played a significant role in the abolitionist movement of the early nineteenth century. He advocated for the rights of African Americans and fought against slavery.