Issue: Dating an event, when sources do not agree about the time periods, is difficult.
Method: Try to minimize the number of competing sources. eg. by eliminating less credible ones.
Result: If this is done, the historians can use the sources which are remaining (mostly several of them will still be left) to arrive at the date which is more probably correct. They are sometimes unsuccessful.
INFERENCE? Ok first things first - your question is a VERY good one, and here is where the subtleties of the GMAT are in play. The reason you are expecting to see predictions / recommendations in the answer choices, is because you think the question is asking you to infer something from an argument / claim / recommendation.
In truth, the question stem reads, "most strongly supported by the information above". Now the word information is key here. Whenever we have a question like this, and it looks like there is a recommendation in there, you need to question whether that is truly a prediction or simply a "directive" that is already in use.
For example, if I say that "Nurses should wash their hands. This has been proven to prevent infection." Though I seem to be making a recommendation, this has already been proven, and it is only a general directive which is actually already in effect. Nurses do wash their hands, as it seems to be proven that this prevents infection. I am just reaffirming that they should.
Similarly, though it looks like the author is recommending a strategy for historians, he is actually reaffirming that this is the correct method. We should take this as a fact, such as from a textbook, that is already being used by these historians. There are also a lot of tells in the passage that point to this... "several sources are left, as often happens..." and "...on occasion unsuccessfully..."
This clearly indicates that historians have in the past tried to eliminate sources and seek a new estimation of dates.
I hope this makes sense. Thus, now we will approach the answer choices to infer what historians do, based on these directives in the information.
A. Too extreme. The passage only talks about events that involve multiple, conflicting sources.
B. We can infer on the basis of the recommendation, that in the past, when faced with an event like this, historians would try to eliminate some sources, and try to determine the date, BUT SOMETIMES UNSUCCESSFULLY. Thus, some of these events cannot be dated reliably by historians.
C. This is out of scope, the passage only says "eliminate less credible ones", not to "identify ones most likely to be true". Also there's no way to know if we can even do this or not.
D. Again out of scope and probably untrue.
E. "undermine the credibility of as many sources as possible" No- this is not the aim. You can only try to eliminate the ones that are less credible, you should not try actively to undermine the sources -- too extreme.
Pick B.