The stimulus points out that a history book from centuries ago contains inconsistencies. The stimulus then shares an argument made by some scholars: because there are inconsistencies in the book (premise), the author must have been getting information from multiple sources (conclusion).

Before looking for assumptions in the scholars' argument, it's really important to be clear on what kind of assumptions we are looking for. This is an unusual question stem. Since it talks about the conclusion "following," you might think this is asking for a sufficient assumption — but it is actually asking for a necessary assumption. Remember that the conditional pattern "not A unless B" translates to A → B, meaning B is a necessary condition for A. So when the question stem states, "The conclusion cited does not follow unless (answer choice)", it is asking for an answer choice that is necessary (not sufficient!) for the scholars' conclusion to follow.

Having this kind of conditional logic baked into a question stem is unusual, and probably because this is an older PT. With that out of the way, let's examine the scholars' argument: based on the sole premise that the book contains inconsistencies, these scholars conclude the author relied on multiple sources.

Ironically, this very simple argument structure makes it easy to pre-phrase a sufficient assumption by bridging the premise-conclusion gap (e.g., if there are inconsistencies in a historical text, the author must have used multiple sources), but a little harder to pre-phrase necessary assumptions, because there's not much reasoning to latch on to. One necessary assumption here might be that the inconsistencies don't come from the author's own errors in copying things down, rather than from his sources. But if you find it hard to pre-phrase beyond that, it might make more sense to go to the answer choices and proceed by elimination.