The correct answer is: b) Tibetan.
Tibetan is a language spoken by about 7 million people in Tibet, Bhutan, Nepal, India, and China. It is the official language of Tibet and Bhutan, and it is also spoken by a significant minority in Nepal and India. Tibetan is a member of the Tibeto-Burman language family, and it is closely related to the languages of Ladakh, Sikkim, and Nepal.
The Bhutia and Lepcha communities are both indigenous peoples of the Himalayas. The Bhutia are found primarily in Bhutan and Sikkim, while the Lepcha are found primarily in Sikkim and Darjeeling. Both communities speak Tibetan languages, and they share many cultural similarities.
The other options are incorrect because they are not languages spoken by both the Bhutia and Lepcha communities. Sikkimese is a language spoken in Sikkim, but it is not spoken by the Bhutia or Lepcha communities. Nepali is a language spoken in Nepal, but it is not spoken by the Bhutia or Lepcha communities. Hindi is a language spoken in India, but it is not spoken by the Bhutia or Lepcha communities.