The correct answer is - d. north to Scandinavia.
Charlemagne formed a great and powerful empire, but he never managed to reach to the coasts of Scandinavia. The Elbe River actually is not just the eastern border but also it the northernmost part of Charlemagne's Empire. He never managed to win over the territory of the Danes so that he comes next to Scandinavia and try to win it over from the Swedes and Norse people, more commonly known as Vikings.