In "Uncle Tom's Cabin," the primary issue raised is the cruelty of slavery. The novel exposed the brutal realities and inhumane treatment of enslaved people, which stirred significant public sentiment against the institution of slavery, particularly in the North. It played a crucial role in shaping public opinion and increasing tensions between the North and South, thus contributing to the divisive atmosphere that eventually led to the Civil War.
The other options related to the importance of slavery for economic reasons, the capabilities of the Confederate states to function independently, or the need for slavery in the North are not central themes of the novel. Therefore, the correct response is:
The novel highlighted the cruelty of slavery.