Explain the theory of how cold European weather may have affected mental development of European people.This can be found in the prologue
This is not something that Diamond says is true. In fact, he argues that cold weather did not actually affect the development of Europeans. However, there is a popular theory that Diamond sets out in the Prologue that does make this argument.
The idea is that the cold weather in Europe made life harder for the Europeans. The cold weather forced the Europeans to cope with all sorts of challenges that were not present in the tropics. The idea is that people who live on a tropical island have it easy. They can just pick fruit off the trees every day and so they don't have to figure out things like technology. By contrast, Europeans had to deal with the harsh climate and they had to do things like inventing technology to help them survive.
So the idea is that a harder life made Europeans smarter because they had to figure out ways to cope.