The best answer here is C. Ethnic cleansing.
Holocaust is associated with another war, the Second World War.
Ethnic diversity is a concept that goes against the actions of Serbian nationalists: they wanted less diversity, not more.
Also, Depopulation is also connected with what the nationalists did, but since they didn't attack all people, but only people of certain (other) ethnicity, Ethnic Cleansing is the best answer.
