"Uncle Tom's Cabin" by Harriet Beecher Stowe played a significant role in changing the attitudes of Northerners about slavery, particularly by prompting them to view it as a moral problem. The novel highlighted the deep injustices and inhumanity of slavery, which stirred the conscience of many readers and fostered a greater sense of empathy and urgency regarding the abolitionist cause. Therefore, the correct response would be:
They began to view it as a moral problem.