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This Logical Reasoning question is from the October 2004 LSAT.
First, let's make a chain of short phrases so we can see how this argument's logic progresses:
Top layer has bones of eaten birds --> lower layers, before humans, have more kinds of bones --> humans caused extinction of many species
Do you see the jump here, the place where the logic breaks down? It's between the 2nd and 3rd phrases. It's saying that just because the number of species decreased when humans arrived that humans are responsible for this decrease. This is a common flaw in LSAT arguments...correlation doesn't equal causation. Just because they (humans' arrival and extinctions) happened at the same time doesn't mean the former caused the latter.
This question asks us to find an answer choice that will weaken the argument. Where's the weak point? It's the jump between correlation and causation. So, we can pre-phrase a rough idea of what kind of answer we need...something that will weaken the argument by casting doubt on that correlation-causation. So, let's go through the answer choices and find one that does that:
A) This is the opposite of what we want, it would strengthen the argument. If the bird didn't have many predators, then it seems more likely that humans' arrival caused its extinction.
B) Again, the opposite of what we want, since it strengthens the argument. If the birds remained longer on uninhabited islands, it is more likely that the humans' presence caused their extinction on this particular island.
C) Correct. This provides a very good reason to think that humans' arrival didn't cause the extinction and thus weakens the argument. If a deadly disease came right before the humans, it could've easily wiped out the birds and the humans may have had nothing to do with it, despite arriving only shortly before the extinction.
D) Beyond the question's scope. Of course bird bones were found...some birds always died. This answer choice does nothing to address why the bird population became less diverse, though, which is what we need.
E) Outside the question's scope. That they didn't fly well might explain eventual extinction (because it made them easier for predators to kill), but it doesn't explain why these extinctions coincided with the arrival of the humans, and thus doesn't weaken the argument as we need it to.
Remember:
1) Make a chain of phrases to show how the argument progresses and find the jump, the place where the logic breaks down. Once you've done a lot of LSAT practice, you probably don't have to actually write this down for most questions other than the very hard ones.
2) Pre-phrase an answer, if you can think of one within a reasonable period of time (since this is meant to save time, not waste it), to the question. This doesn't have to be written down on your paper, usually. It's just a way of finding the right answer choice more quickly instead of plodding through the wrong answers.
3) Practice a lot on weaken questions, as well as strengthen ones. They're very common on the LSAT. Also remember the common issue that correlation doesn't equal causation. It comes up a lot on this test.
4) Find answers that are the opposite of what the question asks for and are beyond the question's scope and eliminate them. This can eliminate most or all wrong answer choices easily.
