Slavs
Note: Besides ethnic groups, Slavs often identify themselves with the local geographical region in which they live. Some of the major regional South Slavic groups include: Zagorci in northern Croatia, Istrijani in westernmost Croatia, Dalmatinci in southern Croatia, Boduli in Adriatic islands, Vlaji in hinterland of Dalmatia, Slavonci in eastern Croatia, Bosanci in Bosnia, Hercegovci in Herzegovina, Krajišnici in western Bosnia, but is more commonly used to refer to the Serbs of Croatia, most of whom are descendants of the Grenzers, and continued to live in the area which made up the Military Frontier until the Croatian war of independence, Semberci in northeast Bosnia, Srbijanci in Serbia proper, Šumadinci in central Serbia, Vojvođani in northern Serbia, Sremci in Syrmia, Bačvani in northwest Vojvodina, Banaćani in Banat, Sandžaklije (Muslims in Serbia/Montenegro border), Kosovci in Kosovo, Bokelji in southwest Montenegro, Trakiytsi in Upper Thracian Lowlands, Dobrudzhantsi in north-east Bulgarian region, Balkandzhii in Central Balkan Mountains, Miziytsi in north Bulgarian region, Warmiaks and Masurians in north-east Polish regions Warmia and Mazuria, Pirintsi in Blagoevgrad Province, Ruptsi in the Rhodopes etc.

Q: Serbs of Croatia are mostly descendants of who?
the Grenzers