I like this quiz idea, but there are several errors. The Volga is the longest river in European Russia, but there are several longer rivers in the Asian part of Russia, with the Yenisey-Angara being the longest system and the Lena being the longest individual river. The Loire is the longest river entirely within France, but the Rhine, which is longer, forms part of France's border with Germany. The Ganges is shorter than the Brahmaputra, which flows through the far eastern part of India, and both are shorter than the Indus, which flows through part of the Indian-controlled part of Kashmir.
In addition to bluesdjben, as a Bulgarian, I have to say that Danube is the northern frontier of Bulgaria. It does not pass through it. Therefore, Iskar river is the longest.
The question is which is the longest river to flow through the country - not which is the longest river to be entirely within that country. So Tagus is correct - just that part of it is in Portugal.