The correct answer is the Rhine.

• The busiest navigable river in the world is the Rhine river.

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

• Hwang-Ho
  • The river Hwang Ho is also known as the 'Sorrow of China'.
  • Because it often used to change its course after floods.
  • This caused major damage to crops and canals dug for the purpose of irrigation.
• Seine
  • The Seine is France's third-longest river, extending 485 miles from Dijon to the English Channel.
  • The river has been the subject of many famous impressionist paintings.
  • Famous impressionist paintings include Monet's Bathers at La Grenouillere and Renoir's Seine River at Asnieres.
• Loire
  • The Loire is the longest river in France.
  • The Loire River was designated a UNESCO World Heritage Site in 2002.