Final answer:
The Underground Railroad was a significant clandestine operation in American history, assisting enslaved African Americans to flee to freedom with the help of abolitionists and sympathetic individuals.
Step-by-step explanation:
The significance of the Underground Railroad in American history is best represented by option 1) It was a network of secret routes and safe houses used by enslaved African Americans to escape to free states and Canada. This network was not a literal train running underground but a covert operation consisting of meeting points, secret routes, transportation, and safe houses coordinated by abolitionists and sympathizers. The most active period of the Underground Railroad was between 1850 and 1860, where it is estimated that approximately 100,000 slaves escaped to freedom. Iconic figures such as Harriet Tubman became symbols of this movement, helping numerous slaves find their path to freedom. Despite the dangers, these 'conductors' of the Railroad, including a diverse group of free-born blacks, white abolitionists, former slaves, and religious organizations such as Quakers, played a crucial role in undermining slavery and pushing the nation towards abolition.