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

Answer

Canada and the United States of America

(a) Canada–United States of America: This is the longest international land border in the world, about 8,891 km including the Alaska boundary. Verdict: Correct.
(b) Chile–Argentina: Their border is very long, about 5,300 km, but clearly shorter than Canada–USA. Verdict: Incorrect.
(c) China–India: The disputed boundary is roughly 3,400–3,500 km, much shorter than the longest border. Verdict: Incorrect.
(d) Kazakhstan–Russian Federation: This is also one of the world’s longest borders, about 7,600 km, but still shorter than Canada–USA. Verdict: Incorrect.

Therefore, the correct answer is (a).