Kangchenjunga is the highest mountain in India (On the border with Nepal) Nanda Devi is the highest mountain situated completely within India (2nd overall) Himalayas is the highest mountain system in India. (Mountain Range)
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.
No, the Andes mountains run along the West coast of South America (Peru, Chile, Edcuador). The Hindu Kush, a mountain range in Northern India and Afghanistan, may be the mountain range you're looking for.