What is an assumption historians make when they evaluate a source of information?

They know that the experts give the most accurate information about the event or the historical process. Explanation:

Historians usually assume that the source of information they are evaluating is the most accurate because they consider that a source of information written by an expert, a researcher or an eyewitness reporter for example is most likely to be correct because they have studied a lot about the specific topic or because they experienced the particular event.

d. the right to establish a one-house legislature

yup! you are correct it is b.