The correct answer is: B. Tharu
The Tharu are an ethnic group native to the Terai region of Nepal, India, and Bangladesh. They are the largest ethnic group in the Terai region, and are estimated to number over 10 million people. The Tharu are a diverse group, with a variety of languages and cultures. However, they share a common history and culture, and are united by their common identity as Tharu.
The Tharu are believed to have originated in the Terai region, and have been living there for centuries. They are a traditionally agricultural people, and their culture is closely tied to the land. The Tharu are also known for their unique music and dance traditions.
In recent years, the Tharu have been facing a number of challenges, including land encroachment, deforestation, and discrimination. However, they are a resilient people, and are working to preserve their culture and identity.
The other options are incorrect because:
- Option A: The Oraon are an ethnic group native to the Chota Nagpur Plateau in India. They are not the largest or oldest ethnic group of the Terai region.
- Option C: The Santhal are an ethnic group native to the Jharkhand and West Bengal states of India. They are not the largest or oldest ethnic group of the Terai region.
- Option D: The Munda are an ethnic group native to the Chota Nagpur Plateau in India. They are not the largest or oldest ethnic group of the Terai region.