The correct answer is option 2 - Caucasus. The Caucasus Mountains are located between the Black Sea and the Caspian Sea. These mountains form a natural boundary that separates the two bodies of water. The Caucasus Mountains are a range that spans a region in Eurasia, including parts of Russia, Georgia, and Azerbaijan.

Option 1 - Urals, are a mountain range that stretches through Russia, but they do not separate the Black Sea and Caspian Sea. The Urals actually separate the European and Asian parts of Russia.

Option 3 - Carpathians, are another mountain range in Europe, but they are not located between the Black Sea and Caspian Sea.

Option 4 - Balkan Mountains, are a range in Southeastern Europe, but they are also not situated between the Black Sea and Caspian Sea.

In conclusion, the Caucasus Mountains are the correct mountains that separate the Black Sea and Caspian Sea.