The correct answer is: a) Guru Padmasambhava (Guru Rinpoche).
Guru Padmasambhava, also known as Guru Rinpoche, was a Buddhist master who is credited with introducing Buddhism to Sikkim in the 8th century. He is also considered to be the founder of Tibetan Buddhism.
Ashoka the Great was an Indian emperor who ruled from 272 to 232 BCE. He is known for his Buddhist reforms and for spreading Buddhism throughout India.
Kanishka was a Kushan emperor who ruled from 127 to 150 CE. He is known for his patronage of Buddhism and for spreading Buddhism throughout Central Asia.
Harshavardhana was an Indian emperor who ruled from 606 to 647 CE. He is known for his military conquests and for his patronage of Buddhism.
While all of these figures were important in the spread of Buddhism, it was Guru Padmasambhava who is credited with introducing Buddhism to Sikkim.