Q. The longest border between any two countries in the world is between : (UPSC Prelims 2024)
Answer: Canada and the United States of America
Notes: The correct answer is [A] Canada and the United States of America. Known as the "International Boundary," it is the longest international border in the world between two countries.

• Canada and USA (Statement [A] – Correct): The border stretches approximately 8,891 kilometers (5,525 miles). It includes the boundary between the contiguous United States and Canada, as well as the border between Alaska and Western Canada. It is often referred to as the "longest undefended border" in the world.
• Kazakhstan and Russian Federation (Statement [D] – Incorrect): This is the second-longest continuous land border in the world, measuring about 7,512 kilometers. While massive, it remains significantly shorter than the US-Canada border.
• Chile and Argentina (Statement [B] – Incorrect): This is the third-longest border in the world and the longest in South America, stretching approximately 5,308 kilometers along the Andes mountains.
• China and India (Statement [C] – Incorrect): The border between India and China (Line of Actual Control) is approximately 3,488 kilometers long. It is divided into three sectors: Western, Middle, and Eastern.

Geographically, the US-Canada border crosses diverse terrains, including the Great Lakes, the 49th parallel, and the rugged wilderness of the Saint Lawrence River. In contrast, the border between Russia and Kazakhstan is primarily a continuous land boundary across the Eurasian Steppe.