A. in different regions of that federation
B. that was Yugoslavia
C. the fundamental cause has been the very large difference in the quality of life
D. although the dismemberment of the federation
E. is seen more as the result of an ethnic conflict

The first sentence should be the one which introduces the topic. Among the given sentences, only D introduces the topic and talks about dismemberment of the federation. D is followed by B saying which federation is D talking about. B is followed by E, E talks about the reason stating it is because ethnic conflict. E is followed by C which states the real reason, i.e. large difference in quality of life. Therefore, the order is DEBCA.