Explanation:

- Option 1: Volga

- The Volga River is the longest river in Europe.

- It flows entirely within Russia and does not pass through multiple countries.

- Option 2: Elbe

- The Elbe River primarily flows through the Czech Republic and Germany.

- It does not travel through ten different countries.

- Option 3: Rhine

- The Rhine River travels through or along six countries: Switzerland, Liechtenstein, Austria, Germany, France, and the Netherlands.

- It is not the second longest river in Europe.

- Option 4: Danube (Correct Answer)

- The Danube River is Europe's second longest river.

- It flows through or borders ten countries: Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova, and Ukraine.

- It travels from the Black Forest in Germany to the Black Sea in Romania.