The correct answer is: a) Guru Rinpoche (Padmasambhava).
Guru Rinpoche, also known as Padmasambhava, was an Indian Buddhist master who is credited with introducing Buddhism to Tibet in the 8th century. He is also considered to be the founder of Tibetan Buddhism.
The Namgyal Dynasty was a Tibetan dynasty that ruled Sikkim from the 17th century to the 1970s. They were not involved in the introduction of Buddhism to Sikkim.
Nepali migrants began to settle in Sikkim in the 18th century. They brought their own Hindu religion with them, and did not play a role in the introduction of Buddhism to Sikkim.
Chinese missionaries did not arrive in Sikkim until the 19th century. They also did not play a role in the introduction of Buddhism to Sikkim.
In conclusion, the introduction of Tibetan Buddhism to Sikkim is largely attributed to Guru Rinpoche (Padmasambhava).