Lets analyze the answer choices and see which one stands scrutiny. To find the choice which weakens the argument, we need to select the choice that provides evidence that the birds' range will not be extended.
(A): The keyword here is 'still'. If there are years now when the chicks are being killed by early snow, has it wiped out the birds? No. Therefore if conditions are favorable it cannot act as an impediment to their range expansion. Incorrect.
(B): Guillemots have been co-existing with their predators where they currently live. If the predators migrate northwards, why will this stop the birds from doing so too? Incorrect.
(C): If temperatures are rising then they will be higher than they are now in inland areas too, possibly allowing the birds to live there. Incorrect.
(D): CORRECT. If the ice in the southern arctic disappears, the birds will be forced to move northwards. However, their RANGE will not be extended as they will just shift from a more southern to a more northern point. There is no evidence that says that they will live over longer stretches of land.
(E): This means there will be more fish for the birds to eat in the North. This supports the conclusion that they will move north and extend their range.
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