Final answer:
The arguable claim is that 'Universal health care is a basic human right' as it is a statement open to debate and interpretation, unlike the other options which are verifiable facts. The correct option is B.
Step-by-step explanation:
The claim that is arguable is B. Universal health care is a basic human right. This is a statement that can be debated and is a matter of opinion and perspective, rather than an objective fact. Claims A, C, and D are factual statements that can be verified with data or historical record. The number of Republicans and Democrats in the Senate, the length of Franklin D. Roosevelt's presidency, and the decade in which Social Security was established are all matters that can be confirmed by checking records and therefore are not open to argument in the same way as the claim about universal health care.