Which two countries share the longest international border?

The option naming Canada and the United States is indeed the correct choice because they share the longest international border in the world, which stretches approximately 8,891 kilometers (5,525 miles). This extensive boundary runs from the Atlantic Ocean in the east to the Pacific Ocean in the west and includes a variety of terrains such as mountains, rivers, and forests, reflecting both countries' geographical diversity.

In contrast, other pairs of countries listed have significant international borders but do not surpass the length of the Canada-U.S. border. For example, the border between China and India is notable for its geopolitical significance and terrain but measures around 3,380 kilometers (2,100 miles). Similarly, while Brazil and Argentina share a border of about 1,261 kilometers (783 miles), it is considerably shorter. The border between Russia and Kazakhstan is lengthy as well, at about 7,591 kilometers (4,735 miles), but it still remains shorter than the Canada-U.S. border.