I agree that it would also be D, because the other answers would be just ways to see a scenario and make it best fit for you, whereas D points out what most extremists do.
It would be between increased population, a steady stream of agricultural production, and technological advancements. As for the most important one of those three; that would be agricultural production.
Are you asking what it is?
Answer:
Cultural diffusion is when one part of a culture moves to another. I think cultural diffusion works when cultures become trade partners. A trait can travel over great distances because of travel. I think one cultural trait that would be passed is a language.
or
Cultural diffusion is the spread of cultural beliefs and social activities from one group of people to another. A trait could travel great distances due to migration to another place. Traits that can be passed down are food, religion, and clothing.
Hope this Helps!