The correct answer is - True.
Even though, we are practically speaking about the same people, with same origin, and same language, there has been rivalries and hatred between them for centuries. In general, the biggest problem between them was created by the religion. The Croats are Catholic, the Serbs are Orthodox, and on top of it we have the population in Bosnia that was Christian, but converted to Islam under the pressure of the Ottomans. Big rivalries came out from this, and unfortunately, the conflicts between them ended up with genocides, the Croats killed Serbs in the World War II, than the Serbs in the early 90's took revenge and were killing Croats, and in the middle of it were the Muslims which also suffered genocide over them.