As far as I'm concerned, there were Serbian communities on Cyprus before it became an EU member. I'm more surprised not to see other ex-Yu countries, they all must have over 100 000 citizens of Serbian nationality.
How is Bosnia and Herzegovina not a correct answer here? It may not count as a diaspora, but the description reads "Name the countries with the most ethnic Serbians".
Wikipedia has about 1 000 000 ethnic Serbs living in Bosnia (source: https://en.wikipedia.org/wiki/Serbs_of_Bosnia_and_Herzegovina)
Wikipedia has about 1 000 000 ethnic Serbs living in Bosnia (source: https://en.wikipedia.org/wiki/Serbs_of_Bosnia_and_Herzegovina)