Nathu La was reopened in 2006 for trans-border trade between India and:

The correct answer is (b). Nathu La was reopened in 2006 for trans-border trade between India and China. It is a high-altitude pass in the Himalayas on the border between the two countries. The pass was closed in 1962 after the Sino-Indian War. It was reopened in 2006 as part of a series of confidence-building measures between the two countries. The reopening of the pass has led to increased trade and economic cooperation between India and China.

The other options are incorrect. Bangladesh is a country in South Asia. Bhutan is a landlocked country in South Asia. Nepal is a landlocked country in South Asia.