Which of the following is the busiest navigable river of the world?

Which of the following is the busiest navigable river of the world?
[A]Volga
[B]Seine
[C]Rhine
[D]Danube

Rhine
The Rhine, which flows in Switzerland, Liechtenstein, Austria, Germany, France and Netherlands, is the most important and busiest waterway in Europe. Other busy waterways include Seine and Loire rivers of France, Danube river of eastern Europe and Volga river of Russia.