What type of hypotheses have been extensively tested and confirmed across a wide range of cases?

The answer is law. In the context of international relations and scientific inquiry more broadly, a law refers to a statement that has been rigorously tested and confirmed across many different cases and contexts. Laws are often characterized by their ability to consistently predict outcomes in a variety of scenarios, demonstrating a high level of empirical support.

Unlike propositions, which can be more general and not necessarily tested, or theoretical statements that may still reside at a conceptual level without extensive validation, laws establish firm connections that are accepted due to their reliability. Assumptions, on the other hand, are initial beliefs or premises that may not have been tested at all. Therefore, the term "law" is most appropriate as it indicates a well-substantiated principle in the field of study.