The correct answer for the world`s busiest inland waterway is option 2, the Suez Canal.

St. Lawrence (option 1) is not the correct answer, as it is not an inland waterway but an estuary and gulf system stretching from the Great Lakes to the Atlantic Ocean.

Rhine (option 3) and Danube (option 4) are both major rivers in Europe, but they are not as busy or heavily trafficked as the Suez Canal. The Rhine is an important waterway for transportation and commerce in Western Europe, but it does not carry as much traffic as the Suez Canal. The Danube is the longest river in the European Union, but it is primarily used for transportation within Europe and does not have the same level of traffic as the Suez Canal.

Therefore, the Suez Canal is the correct answer as it is the busiest inland waterway, connecting the Mediterranean Sea and the Red Sea, providing a crucial shortcut for maritime trade between Europe and Asia.