Final answer:
Whigs, Copperheads, and Know-Nothings were all third parties or factions in American political history; therefore, 'all of the above' best completes the sentence.
Step-by-step explanation:
Whigs, Copperheads, and Know-Nothings are all names of third parties. In the mid-19th century, the primary political parties in the United States were the Democrats and the Whigs. After the Whigs disintegrated, several factions emerged, including the Know-Nothings or the American Party, which became prominent by opposing immigration, and the Republican Party, which was concerned about the expansion of slavery. Meanwhile, the Copperheads were a faction of the Democratic Party during the Civil War, known as Peace Democrats who opposed the war.
The correct term that best completes the sentence is 'all of the above.' Each term - Whigs, Copperheads, and Know-Nothings - refers to a third party or faction that played a significant role in American political history at different points in time.