The correct answer to the question is answer choice D. Diamond writes that when the Vikings first came to Iceland, "the landscape looked familiar and encouraging." It superficially resembled the landscape they left behind in Norway and thus appeared to be capable of supporting the pastoral and agricultural lifestyle they had developed in Norway and the British Isles. But, he writes, these appearances were deceptive in "three crucial respects." First, Iceland's higher latitude (relative to Norway) meant that the climate was cooler and the growing season was shorter (answer choice A). Second, "the ash from volcanic eruptions periodically ejected over wide areas poisoned fodder," which made it more difficult to raise the livestock that the Vikings raised in Norway and Britain (answer choice B). Finally, though Iceland had, like Norway, rich soils, these soils "form more slowly and erode much more quickly" than those in Norway. In other words, they were more fragile, which is answer choice C (198). All of this meant that Iceland's settlers sought to impose a lifestyle on an environment that was, despite initial appearances, incapable of supporting it. They found, in short, "an apparently lush but actually fragile environment for which their Norwegian and British experience had failed to prepare them" (201). As a result, Iceland was for many centuries one of the poorest places in Europe, and its people did great damage to their environment. Answer choices A, B, and C were each issues confronted by Norse settlers in Iceland.

The answer to this can be found in Chapter 6 of Collapse.  Since I only have this book on Kindle, I cannot give you a page number and Amazon’s searchable copy does not include the relevant pages in the sections that are viewable without purchasing.  The best answer to this question is Option D: all of the above.  All of these are listed in Chapter 6 as problems that Icelanders faced.

In Chapter 6, Diamond tells us that

…Iceland’s apparent similarity to southwestern Norway and Britain was deceptive in three crucial respects.

When he goes on to list these “crucial respects,” we see that they are all of the things that are mentioned in your question.  First, he says, Iceland was farther north.  This meant that it had a “cooler climate and a shorter growing season.”  This means that Option A from your question is correct.  Second, Diamond says,

…ash that volcanic eruptions periodically ejected over wide areas poisoned fodder for livestock.

He says that this has caused starvation at various points in Iceland’s history.  Therefore, Option B is also correct.  Finally, Diamond says that the biggest problem is the fact that Norway and Britain had “robust” soils whereas Iceland’s soil was “fragile.”  This means that Option C is also correct.

Therefore, Option D must be the correct answer to this question.