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.