The McMahon Line forms a part of the border between Arunachal Pradesh and:

The correct answer is: a) China (Tibet).

The McMahon Line is a 2,400-kilometre (1,500 mi) border between India and China, running from the trijunction with Myanmar in the east to the trijunction with Bhutan in the west. It was established in 1914 by the Simla Convention between British India and Tibet, but China did not recognize the convention. The McMahon Line is disputed by China, which claims that Arunachal Pradesh, the Indian state located north of the line, is part of its territory.

Myanmar is located to the east of Arunachal Pradesh, Bhutan is located to the west, and Nepal is located to the south.