The correct answer is: a) Early settlers from the Tibetan plateau.
Yak herding is a traditional practice in parts of Sikkim, likely has connections to early settlers from the Tibetan plateau. Yaks are well-adapted to the high altitude and cold climate of the Tibetan plateau, and they have been used for transportation, milk, meat, and wool for centuries. The practice of yak herding is likely to have been brought to Sikkim by early settlers from the Tibetan plateau.
Option b) Migration patterns from Nepal is incorrect. Yak herding is not a traditional practice in Nepal, and there is no evidence to suggest that it was brought to Sikkim by migrants from Nepal.
Option c) British colonial influence is incorrect. The British did not have a significant impact on the practice of yak herding in Sikkim. Yak herding was already a well-established practice in Sikkim before the British arrived.
Option d) Recent agricultural innovations is incorrect. Yak herding is not a recent innovation. It is a traditional practice that has been in existence for centuries.