The correct answer is - Danube.
The Danube River is the second longest river in Europe, being only shorter by the Volga River. This river starts its journey in the southern part of Germany, in the Black Forest, and ends its journey in the Black Sea. From its begging to its end, the Danube passes through or touches the borders of multiple countries, ten countries to be more specific, including Germany, Austria, Slovak Republic, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova, and Ukraine.