This is a Cause & Effect argument. The phenomenon that the author is trying to explain is why a volcanic boulder is in an area where the rest of the rock is sedimentary.
The proposed cause (i.e. the conclusion)? That the boulder was probably deposited here by a glacier. The support provided for this conclusion is that this area was covered by southward-moving glaciers during the last ice age.
Notice that for the author's conclusion to be true, there would have had to be volcanic rock north of where this boulder was found. Otherwise, how could a southward-moving glacier serve as the explanation for its presence here?
We are looking for an answer choice that "most seriously undermines" this conclusion so this is a Weaken question.
(D) states that there are no geological sources of volcanic rock north of this boulder.
This weakens the argument because, as stated above, the author's conclusion is based on the premise that a southward-moving glacier brought the volcanic boulder, which would mean that volcanic rock would have to have been located north of the location. (D) says there were not which clearly weakens the conclusion.
Hope this helps! Please let us know if you have any other questions.
