There are two main mountain ranges in ranges in Bulgaria, the northern one is called the Balkan Mountains and the southern one is called the Rhodope Mountains.
Vrah Musalla ( peak Mussala) in the Balkan mountain
According to various websites, the best mountain in Bulgaria for skiing, is the Vitosha mountain. The Vitosha mountain is situated close to the capital of Bulgaria.
Rila - 2925m
I think it is the mountain Rila.
Balkan mountain range
Varna
Its the Danube river and Danube plain separates Romania and Bulgaria. There are no mountaneous border between the mentioned two countries. And Carpathian mountain range of Romnia or Balkan mountain range (most northern mountains of Bulgaria: none of them serves as border.
The Balkan Mountain range (in Bulgaria Balkan Mountain is often called the 'Stara Planina' Mountains)
The Stara planina mountain (Стара планина)
Stara Planina
Balkan mountain range, The Daniube and a capital Sofia!
the stara planina mountains
The Balkan mountain range runs from the border with Serbia 560km eastwards to the Black Sea
A common mountain range covering Bulgaria and Serbia is called the Balkan Mountains (in Bulgarian also called 'Stara Planina')
The Summit of the Rila Mountain range Musala is at height-9596 feet
Balkan Mountain ranges (Stara Planina), biggest range covering Bulgaria and Serbia, Rhodopes range (Rodopi), Rila, Pirin. and may be Vitosha (some includes it Balkan range!)