Final answer:
The Po River is the longest river in Italy and forms a large delta in the Adriatic Sea. It runs eastward from the Cottian Alps for about 652 kilometers before reaching the sea.
Step-by-step explanation:
The longest river in Italy that forms a large delta in the Adriatic Sea is the Po River. As the main river draining into the Adriatic Sea, the Po River has significant historical and geographical importance to the region.
Originating in the Cottian Alps, the river runs eastward for about 652 kilometers (405 miles), passing through many important Italian cities before it reaches the Adriatic Sea and forms a delta. This delta is not only a productive agricultural area due to the rich sediment the river carries, but it is also a critical habitat for wildlife, recognized as a UNESCO Biosphere Reserve.
While the Amu Darya and the Nile River are mentioned as the longest rivers in their respective regions, and the Danube River is described as a significant European waterway forming an invasion corridor, they are not related to the query about Italy's longest river. Moreover, the information shared is used to contrast with other rivers worldwide, emphasizing the unique characteristics and roles of the Po River in Italy.