The answer to this can be found in Chapter 15 of Guns, Germs, and Steel. Specifically, it can be found beginning on p. 308 of the paperback edition of the book. When thinking about the answer to this question, we should remember the basic thesis that Diamond is putting forward in this book. His basic argument is that geography is the determining factor for which areas get agriculture and which do not. Because of this, Option B is the correct answer. In fact, the wording of Option B is almost exactly the same as the wording on p. 308.
According to Diamond, animal husbandry was not going to work in Australia. The problem is that Australia essentially had no animals to domesticate. Diamond says that all of the
…candidates for animal husbandry disappeared in the wave of extinctions (or exterminations) that accompanied human colonization of Australia.
No domesticable animals means no animal husbandry.
Diamond also tells us that Australia was not very good for agriculture. One problem is that Australia is
…not only the driest continent but also the one with the most infertile soils.
He later goes on to tell us that Australia had very few plants that were good for domestication. In these ways, we can see that Diamond would say that Option B is the correct answer.