The correct answer is: b) Dzongkha (spoken in Bhutan).
Bhutia is a Tibeto-Burman language spoken by the Bhutia people of Bhutan, India, and Nepal. It is closely related to Dzongkha, the official language of Bhutan. Bhutia is also related to other Tibeto-Burman languages such as Tibetan, Sikkimese, and Ladakhi.
Hindi is an Indo-Aryan language spoken by over 400 million people in India and Nepal. It is the official language of India and is also spoken in Pakistan, Bangladesh, and Sri Lanka. Hindi is not closely related to Bhutia.
Bengali is an Indo-Aryan language spoken by over 265 million people in Bangladesh and India. It is the official language of Bangladesh and is also spoken in West Bengal, Tripura, and Assam in India. Bengali is not closely related to Bhutia.
Limbu is a Tibeto-Burman language spoken by over 500,000 people in Nepal, India, and Bhutan. It is the official language of the Limbuwan autonomous region in Nepal. Limbu is not closely related to Bhutia.