Bulgaria.
Serbia and Bulgaria
Bulgaria and Serbia.
Europe, on the Balkan peninsula (Serbia and Bulgaria).
The Balkan Mountain range (in Bulgaria Balkan Mountain is often called the 'Stara Planina' Mountains)
Serbia
In SE Europe. From NE Italy running south of the Danube through Croatia, Bosnia & Serbia to Bulgaria & Macedonia.
Hungary, Serbia, Bulgaria, Moldova, Ukraine (and the Black Sea).
If you do not consider Kosovo to be a part of Serbia, then the answer is: Macedonia and Bulgaria. If you do consider Kosovo to be a part of Serbia, the answer is: Albania, Macedonia, and Bulgaria.
Macedonia shares its borders with Serbia, Albania, Greece and Bulgaria
Serbia
Romania borders Bulgaria, Serbia, Hungary, the Ukraine, and Moldova.