Ed M. answered 03/05/16
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Help with grammar, French, SAT Writing, the TOEFL and ESL.
The events and conflicts that transpired in the 1990s following the dissolution of the Socialist Federal Republic of Yugoslavia were quite complex, but the Republic of Serbia, particularly under the government of President Slobodan Miloševic, was most often blamed for conducting territorial aggression--and concomitant war crimes--in that region.
So probably the best answer to your question among the choices you give is "a.serbia [sic]," but again I do think many people would have a problem with characterizing Serbia as desiring "to rule the entire country," in the senses that (1) one could argue that the primary objective of the Serbian government was to protect ethnic Serbs in other areas of the former Yugoslavia, and (2) if there was any motivation to dominate and/or incorporate the other nations it may not necessarily have been any stronger in Serbia than in the other three you mention.
Plus, I believe your question is a bit circular in itself since if Yugoslavia is a "former" country, how could any other nation "rule the entire country"?