The danube river is an important transportation corridor because it
a) connects four major european cities.
b) contains many natural resources.
c) attracts many foreign tourists.
d) flows from north to south.

The Danube is the second longest river in Europe flowing from Germany to the Black Sea passing through ten different countries. It is a major water supply for irrigation, crops and drinking water for millions of people, and is used as a major shipping route and for cultural tourist destination. (A) is correct as it passes through Budapest, Vienna, Bratislava and Belgrade (all capital cities), (C) is correct as there are many tourists travel to the river on cruises, (B) could be correct also, however its main natural resource is water, and its route, so I would not consider it to have "many" natural resources. And (D) would not be correct as the river flows West to East not North to South.

A) connects four major European cities.

Explanation:

Being Europe's second most widespread and largest river system it flows through the 10 counties and originates in Germany and drains into the black sea. It is a good feeding habitat for the fish population and has a large diversity. As of today, about  2,415 km area is navigable through this rover system hence it's mostly used in the process of translation and a major source of hydropower in Europe. Danube river delta is the largest river delta in the EU and this river is much older than other rivers of Europe thus the Danube is an important transport route.