IMO A
it is a simple premise and conclusion structure.
Premise: projectile did not resemble to any projectile found in the part of eurasia closest to america.
simpler terms, let say well known place India. since the projectile did not resemble to any projectile found in bombay, the author states that the projectile must have come from Chennai(distant part of eurasia).

Conclusion:First Eurasian settlers in North america came from distant part of Eurasia.
Assumption: Projectile found resembled the projectile in distant part of Eurasia.

A) weakens this statement perfectly by telling that the projectile does not resemble any projectiles in the country. So it just means that Bombay/Chennai/ Delhi. it does not matter. It does not belong to country at all. so no further conclusion as to from where the first eurasian settlers came cannot be drawn.

B)simpler terms, people at Bombay are nomadic. So does it weaken the statement that the first settlers came from Chennai?
Does it weaken that the assumption??Hence wrong.

C D and E are simply out of scope!

Hope this helps!

IMO A

Yes, the correct answer is definitely choice A, and fibonnaci's reasoning is good.

The classic Kaplan method will have saved us a lot of time. Because the assumption bridges the gap between the ideas in the evidence and the idea in the conclusion, the classic Kaplan method involves looking for ideas that are differentially present in conclusion and evidence.

Notice a big idea in the evidence is this whole "projectile" business:

"it is a simple premise and conclusion structure.
Premise: projectile did not resemble to any projectile found in the part of eurasia closest to america."
(fibonnaci's post--quote attempt failed)

But notice this idea or term "projectile" is NOT in the conclusion:

Conclusion:First Eurasian settlers in North america came from distant part of Eurasia.

Thus, the arguer is definitely assuming something about projectiles:

Assumption: Projectile found resembled the projectile in distant part of Eurasia.

We weaken arguments by finding a choice that attacks the assumption. In order to attack the assumption, an answer choice has to be within the scope of the argument (since the assumption is always within the scope of the argument). Accordingly, the correct answer almost certainly has to have the term "projectile" in it. A quick scan of the answer choices reveals that only choices A and C have the term "projectile" in them. And choice C is definitely outside the scope, so choose A...done!

(The method involves generating a prediction and aggressively scanning the answer choices for a match. If you get good at it, you can say: "if this is my prediction and if it is correct, what features of my prediction almost for sure have to show up in the right answer" and then you just scan for those features. Here, the key feature or term or idea was "projectile".)