Answer:
The claim that is arguable is B: Senators should not have term limits.
Step-by-step explanation:
I chose B because it is the only claim that contains a personal opinion. All the others are facts: there are more Democrats that Republicans in Congress, the Constitution divides powers between the federal government and state governments, and the United States has 50 states. All of these are facts, but the second one (B) is a personal opinion, and we realize that when reading the word should, therefore is arguable because another person might have a different opinion about the same subject.