The answer to this question can be found in Chapter 16 of Guns, Germs, and Steel. Specifically, it can be found beginning on p. 331 of the paperback edition of the book. Of these options, the best answer is Option B.
In general, regions with long north-south axes do not do as well as regions with long east-west axes. This is because crops can diffuse better along east-west axes. China’s longer axis is north to south and therefore China should have been harder to unify than some other regions. However, there are two main factors that allowed China to unify.
First, as Diamond says,
…China’s long east-west rivers … facilitated diffusion of crops and technology between the coast and inland…
This language is almost exactly the language of Option B. Therefore, Option B is the best answer.
There is one other factor that made China easier to unify. This is similar to Option D, but not exactly the same. Diamond tells us that China had “relatively gentle terrain” on the north-south axis between the two east-west rivers. That gentle terrain allowed the Chinese to build canals between the two river systems. This “facilitated north-south exchanges.” Thus, there is gentle terrain, just as Option D talks about, but the terrain was on the north-axis.
For these reasons Option B is the best answer.