The correct answer is - C. Caspian.
The region of Eastern Europe, can be said that it is located north of the Caspian Sea, but in reality that is only one part of the whole region, while the rest will be excluded by that logic. North of the Caspian Sea is the territory of Russia, and Russia takes up a very big portion of Eastern Europe, but the rest of the countries will be excluded, especially the Balkan countries that also fall into this region, but are in general more on the south from the Caspian Sea.