The correct answer is B) cultural differences.
Population distribution in Russia and Europe is most impacted by cultural differences.
Such that has been the case since World War 1, where there were so many cultural and political differences between the nations that forced them to create alliances to protect their interest, such as the Triple Entente (France, Great Britain, and Russia) and the Triple Alliance (Germany, Italy, Austria-Hungary). Then after World War II, cultural differences and political decisions divided again the map of Europe. The Cold War also was a heavy influence in the split decisions.
The Soviet Union dissolution is another demonstration of what cultural differences can do between formerly countries, as well as what happened with the Czech Republic, Slovakia, and Yugoslavia.
