Final answer:
Sumner's opinion of the Dred Scott decision can be characterized as one of the worst decisions in history.
Step-by-step explanation:
The best characterization of Sumner's opinion of the Dred Scott decision is that he believes it is one of the worst decisions in history (option 3). The decision denied Dred Scott's claim to freedom and declared that Black people could not be citizens. Sumner strongly disagreed with these aspects of the decision, as it perpetuated and protected the institution of slavery. The Dred Scott decision contributed to the sectional divide between the North and the South, further intensifying the debate over slavery.