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I would appreciate some help on this one. I can see how (A), the correct answer, weakens the argument, but I think that (E) weakens the argument as well

Answer choice (E) states: Climatic conditions in North America just before the Ice Age were more conducive to human habitation than were those in the part of Eurasia closest to North America at that time.

Would this not weaken the conclusion in the argument?

Conclusion: "Thus, since Eurasians did not settle in North America until shortly before the peak of the Ice Age, the first Eurasian settlers in North America probably came from a more distant part of Eurasia.'

I think it weakens the conclusion because you are bringing in an alternative to the first Eurasian settlers in North America being "probably" from a more distant part of Eurasia. If it is that the conditions were more conducive in North America, then that affects the likelihood of the settlers' probability, does it not?

I see you line of thinking, and another version of (E) is this: "North America had more fish than the closest parts in Eurasia and humans do better with more fish." The question is whether knowing that there's a reason to move means that the conclusion - that the first settlers probably didn't come from the closest parts of Eurasia - can't be drawn from the fact that the arrowheads found in the mastodon body were not like the ones used in that part of Eurasia.

There's a couple of reasons to eliminate (E). For one, it's about the conditions just before the Ice Age, and in the stimulus we learn that the migration happened shortly before the peak of the ice age. More importantly, the point of a weakener 95% of the time is to disrupt the connection between the premise and conclusion (we're asked to weaken the argument, not the conclusion). At best (and it's not), (E) is a counter to the conclusion.

Notice how when you cited the conclusion, you actually cited the entire argument. I may be off here, but it seems like you should drill down a bit more into the core, separating the premise from the conclusion so that you can focus on the gap.

Edit: Fuller explanation.