Among the options provided, the longest river is the Amazon River. Here`s the detailed reasoning behind this conclusion:

1. Amazon River:

The Amazon River is the largest river by discharge volume in the world and flows through South America. It spans across multiple countries, including Brazil, Peru, Colombia, and others. With a length of approximately 6,400 kilometers (4,000 miles), it is widely recognized as the longest river in the world. The Amazon River is known for its vast drainage basin, which covers an area of about 7 million square kilometers (2.7 million square miles). It carries a massive amount of water, accounting for approximately 20% of the world`s total river flow.

2. Amur River:

The Amur River is located in eastern Asia and forms the boundary between Russia and China. While the Amur River is significant in terms of length, with a total length of around 2,824 kilometers (1,755 miles), it is not as long as the Amazon River. The Amur River is primarily known for its importance in terms of trade, providing a vital transportation route for the region.

3. Congo River:

The Congo River is situated in Central Africa and is the second-longest river on the continent. It has a length of about 4,700 kilometers (2,920 miles). Although the Congo River is impressive in terms of its size and flow, it falls short of the length of the Amazon River.

4. Lena River:

The Lena River is located in Siberia, Russia, and is one of the longest rivers in the world. It stretches approximately 4,400 kilometers (2,734 miles) in length. While the Lena River is notable for its significance in Siberia and its role in transportation, it is not as long as the Amazon River.

In summary, among the given options, the Amazon River is the longest river, stretching over 6,400 kilometers (4,000 miles) and encompassing a vast drainage basin. The Amur, Congo, and Lena rivers, while notable in their own right, do not surpass the length of the Amazon River.