"Multiculturalist relativism” is the notion that there is no such thing as impartial or objective knowledge. The author seems to be grounding his criticism of this notion on:
the conclusion that relativism leaves one with no clear notions of any one thing that is true.
Look at these lines in the passage -
“Impartial knowledge, they argue, is not possible, because ideas are simply the expression of individual identity, or of the unspoken but inescapable assumptions that are inscribed in a culture or a language. The problem, however, with this warmed-over Nietzscheanism is that it threatens to leave no ground for anybody to stand on”
These lines clearly point to (a) as the right answer.