According to Guns, Germs, and Steel, the diffusion of food production occurred more rapidly in Europe and Asia than it did in North and South America because:
a) The East-West axis of Europe and Asia meant that plants adapted to a particular latitude could be more readily cultivated in new regions along that axis.
b) The populations of North and South America were engaged in continual conflict, making diffusion a much slower process.
c) Neither. The diffusion of food production occurred at virtually the same rate among these continents.
d) The population density of Europe and Asia was much higher, leading to greater ease in the diffusion of innovations.
In your question, you do not indicate if more than one of the answers can be acceptable. If you are allowed to give more than one answer, the best answers are A and D. If you are only allowed one answer, A is probably better than D.
B and C are not accurate statements. C is not correct because the diffusion of crops in the Americas was very slow. This is why the “three sisters” from Mexico did not make it to what is now the Eastern United States until something like 900 AD. This is much slower than the diffusion of crops in Eurasia. B is not correct because it is also not true. The peoples of North and South America were hardly even in contact with one another. They were certainly not in conflict.
One of Diamond’s major points in this book is the point made in Option A. Diamond devotes all of Chapter 10 to explaining this idea. Crops do diffuse better from east to west because the climate changes less when going in that direction than it does when going north to south. However, Option D is also correct. When there are more people, more people are in contact with one another and diffusion is easier.
Thus, A and D are both good answers, but Diamond spends more time talking about A in this book so I would choose A if only allowed one answer.