The correct answer is: b) Tibet.
Thangka paintings are a form of Tibetan Buddhist art. They are usually painted on cotton or silk cloth and depict religious scenes or figures. Thangkas are often used in religious ceremonies and are also popular tourist souvenirs.
The regions of Arunachal Pradesh bordering Tibet are the Tawang district and the West Kameng district. These districts are home to a large number of Tibetan Buddhists and many monasteries. Thangka paintings are an important part of Tibetan Buddhist culture and are found in many homes and monasteries in these districts.
The other options are incorrect because they do not border Tibet. Bhutan is located to the south of Arunachal Pradesh, Myanmar is located to the east, and Assam is located to the west.