MacMohan line forms the boundary between

1. A

   India and China

   (a) India and China. The McMahon Line, drawn at the 1914 Simla Convention between British India and Tibet, is the de facto India–China boundary in Arunachal. That stored pair is the official key.

2. B

   India and Pakistan

   (b) India and Pakistan. India–Pakistan lines are Radcliffe, LoC and Sir Creek, not McMahon.

3. C

   India and Myanmar

   (c) India and Myanmar. India–Myanmar follows the Patkai–Arakan hill alignment, not McMahon.

4. D

   India and Nepal

   (d) India and Nepal. India–Nepal is a peace-and-friendship open border, not McMahon.

Summary. Official key is (a) India and China. McMahon Line names Henry McMahon and the Simla Convention of 1914. Pakistan, Myanmar and Nepal borders use other names. Neighbouring places are Tawang and the Thagla ridge. Honour India–China for this paper.