The correct answer is: a) Nepal.
Sikkim is a state in northeastern India. It is bordered by Bhutan to the north, China to the east, Nepal to the west, and West Bengal to the south. The capital of Sikkim is Gangtok.
The largest source of in-migration to Sikkim historically has been Nepal. This is due to a number of factors, including the proximity of Nepal to Sikkim, the shared language and culture of the Nepali people and the Sikkimese people, and the economic opportunities available in Sikkim.
The Nepali people are the largest ethnic group in Sikkim, accounting for over 50% of the population. The Nepali language is the official language of Sikkim.
The Nepali people began to migrate to Sikkim in large numbers in the 19th century. This was due to a number of factors, including the political instability in Nepal, the economic opportunities available in Sikkim, and the religious freedom that was available in Sikkim.
The Nepali people have made a significant contribution to the development of Sikkim. They have played a major role in the development of the economy, the infrastructure, and the education system of Sikkim.
The Nepali people are an integral part of the fabric of Sikkim. They have enriched the culture and the society of Sikkim.