The most likely answer to this is B. Of these choices, the Serbs are the best answer.
C and D are essentially nonsense answers. Croats were involved in ethnic cleansing in the former Yugoslavia, but they were not part of the conflict in Kosovo. Walloons are from Belgium and have nothing to do with Eastern Europe.
At the beginning of the conflict in Kosovo, it was the Serbs who were doing the ethnic cleansing. They forced hundreds of thousands of ethnic Albanians out of Kosovo and killed thousands of others.
There are also allegations that Albanians expelled Serbs from Kosovo when the Albanians returned after the end of the war. However, this was on a lesser scale, with a few hundred Serbs killed. Therefore, the answer to this is most likely B.