The correct answer is: a) Tibet.
The Pala Empire was a Hindu empire that ruled over much of eastern India from the 8th to the 12th centuries. The Kamarupa Kingdom was a Hindu kingdom that ruled over much of Assam from the 7th to the 13th centuries. Nepal is a country in South Asia that has been independent since the 18th century. Tibet is a region in East Asia that has been under Chinese rule since the 18th century.
Tibet is believed to have extended its influence over parts of Sikkim before the rise of the Namgyal dynasty because of its proximity to Sikkim and the fact that Tibet was a powerful kingdom at the time. The Pala Empire and the Kamarupa Kingdom were both located further away from Sikkim and were not as powerful as Tibet at the time. Nepal was not even a country at the time that Tibet is believed to have extended its influence over parts of Sikkim.
The Namgyal dynasty was a Buddhist dynasty that ruled over Sikkim from the 17th to the 20th centuries. The dynasty was founded by Phuntsog Namgyal, who was a Tibetan prince who fled to Sikkim in the 17th century. Phuntsog Namgyal was able to unite the various tribes of Sikkim and establish a stable government. The Namgyal dynasty ruled Sikkim until it was annexed by India in 1975.