The two main ideas of "Harriet Tubman and the Underground Railroad" are:
Tubman is respected among abolitionists for her courage, experience, and knowledge.
During the Civil War, Tubman uses her skills to help the Union army free persons held in slavery.
These two ideas are closely connected and both demonstrate the important role that Harriet Tubman played in the abolition of slavery, both as a conductor of the Underground Railroad and as a scout and spy for the Union army during the Civil War. The other options are not the main ideas of the topic and only gives additional information.