Final answer:
The Danube River acts as the border between Croatia and Serbia, and is an important river in Europe due to its role in trade and geography.
Step-by-step explanation:
The river that flows along the border between Croatia and Serbia is the mighty Danube River.
The Danube is a significant waterway in Europe, flowing from its origins in Germany through various countries including Austria, Hungary, and eventually serving as the border between Croatia and Serbia before continuing its journey eastward and into the Black Sea.
The river has historical significance, acting not only as a natural boundary but also as a conduit for trade and cultural exchanges over the centuries.