Final answer:
The Danube River forms a large delta in Romania which is the second-largest in Europe and empties into the Black Sea.
Step-by-step explanation:
The river that forms a large delta in Romania and empties into the Black Sea is the Danube River. This delta is the second-largest in Europe, following the Volga Delta in Russia. After flowing across several European countries, the Danube River forms a natural border with Bulgaria and Serbia. Its journey ends with the creation of an extensive and biodiverse delta in Romania, known as the Danube Delta, before the river flows into the Black Sea.
Furthermore, the Danube River is significant for its role in transportation and ecology. This river, being one of the main rivers that divide Europe, also connects to the Main and Rhine Rivers, supporting an interconnected network of European waterways.