Answer:
The answer to the question: What has been a serious case of division among the nations of Eastern Europe, would be: A: Ethnicity and C: Religion.
Explanation:
Almost from their very creation there has been a lot of issues that have driven the countries from Eastern Europe into constant conflict. One such issue has been ethnicity, given that there has been, historically, a perception on the inferiority of certain ethnic groups by others, even within the countries themselves. One such example is the ethnic persecutions in conuntries like Serbia, or in Bosnia. Ethnicity, in most of these countries, is seen as a point of conflict, and it has driven a lot of the massacres, and wars, that have ensued. Another issue has been religion, especially given the vast amount of influences that these lands have received throughout history. However, Eastern Europeans do not accept differences of religion, and it is known that they do not accept Judaism, or Islam, despite having had influences from both. Because these religious minorities exist within the countries, many conflicts have arisen when the governments of these Eastern European nations, have wished to eradicate them. This is why these are the two main issues that have caused constant problems in Eastern Europe.