What answer does Diamond propose to Yali's question in the prologue of Guns, Germs, and Steel?
In the prologue, Diamond does not propose a detailed answer to Yali's question. However, he does propose a general answer that he will flesh out in the rest of the book. Yali asks why it is that the white people have so much more wealth and power than his people. Diamond proposes that it was environmental and geographic factors that caused this difference. In doing so, he rejects the sorts of explanations for this difference that rely on race and/or culture.
The best summary of Diamond's argument that can be found in the Prologue is on page 25 (in the paperback edition). There, Diamond says that the differences Yali asks about come to exist
...because of differences among peoples' environments, not because of biological differences among the peoples themselves.