The correct answer is: a) Tibetan Buddhism.
Tibetan Buddhism is a religion that originated in Tibet and spread to other parts of Asia, including Arunachal Pradesh. It is a Mahayana Buddhist tradition that emphasizes the importance of compassion and wisdom. Tibetan Buddhism is characterized by its use of ritual, meditation, and prayer.
The presence of Buddhist ruins and monasteries in Arunachal Pradesh, particularly in regions bordering Tibet, suggests that the region has been influenced by Tibetan Buddhism for centuries. The ruins of these monasteries provide evidence of the early presence of Buddhism in the region, while the continued existence of these monasteries today testifies to the enduring popularity of Tibetan Buddhism in Arunachal Pradesh.
The other options are incorrect because they do not accurately reflect the history and culture of Arunachal Pradesh. Hinduism is the majority religion in India, but it is not the dominant religion in Arunachal Pradesh. The Donyi-Polo faith is an indigenous religion that is practiced by many people in Arunachal Pradesh, but it is not a Buddhist religion. Theravada Buddhism is a branch of Buddhism that is practiced in Southeast Asia, but it is not the dominant form of Buddhism in Arunachal Pradesh.