The correct answer is: a) Sikkim and Tibet (China).
The Nathu La Pass is a high mountain pass in the Himalayas on the border between India and China. It is located in the Indian state of Sikkim, about 56 km east of Gangtok, the capital of Sikkim. The pass is 4,310 meters (14,140 feet) above sea level.
The Nathu La Pass was closed in 1962 after the Sino-Indian War. It was reopened for trade and tourism in 2006. The pass is an important trade route between India and China. It is also a popular tourist destination.
The other options are incorrect because they do not border China. Sikkim is a landlocked state in northeastern India. It is bordered by Bhutan to the west, Nepal to the northwest, China to the north and east, and West Bengal to the south.