So I was stuck between (A) and (B). I incorrectly chose (A).
Is (A) incorrect because an assumption is something always unspoken? So a written statement never be an assumption?
Or is (A) wrong because it is not true that there is no support offered for Joseph's statement "probably this alleged theorem simply cannot be proved, since--as the article points out--no one else has been able to prove it." I suppose the support is the article he mentions?
The reason that (B) is correct seems to be much more straight forward. It essentially supports the final conclusion of Joseph's argument, and it passes the "THEREFORE" test.