There is an important sentence in the passage that I think we should pay attention to: "Equality is presumed by such critics to require unacceptably bland uniformity." This is what the critics' rejection of total equality is based on. Presumption and assumption have roughly the same meaning, and there isn't any evidence given to support the critics' claim. The political scientist attacks this assumption by offering an opposing theory.
The answer choices won't always say "premise," or use the LSAT language that we have become familiar with. A premise is really any piece of information provided by the stimulus. The answer choices are often more specific. In this case, the critics' argument is based on an assumption.