The longest border between any two countries in the world is between :

1. A

   Canada and the United States of America

   Canada–USA. The world’s longest land border.

2. B

   Chile and Argentina

   Chile–Argentina. Long Andean border, not the longest worldwide.

3. C

   China and India

   China–India. Long, disputed, not the global maximum.

4. D

   Kazakhstan and Russian Federation

   Kazakhstan–Russia. Very long, still shorter than Canada–USA.

Summary. Official key is (a) Canada and the United States. That land boundary is the longest between any two countries. The other pairs are long regional borders, not the world record.