Uncle Tom's Cabin significantly influenced the attitudes of Northerners about slavery, primarily by making them view it as a moral problem. The novel, written by Harriet Beecher Stowe and published in 1852, illustrated the harsh realities of slavery and humanized the enslaved characters, which helped galvanize anti-slavery sentiments in the North. Therefore, the correct answer is:
3. They began to view it as a moral problem.