The Rajbongshi or Koch-Rajbongshi is an ethnic group primarily inhabiting the state of Assam in India. They are one of the indigenous communities of the region and have a significant presence in the western districts of Assam, such as Kokrajhar, Bongaigaon, Dhubri, and Goalpara.

The Rajbongshi people have a rich cultural heritage and a distinct language known as Rajbongshi or Kamtapuri. They are believed to have originated from the ancient kingdom of Kamrupa, which was located in present-day Assam. Over the centuries, they have faced various challenges and struggles, including conflicts with other communities and political marginalization.

Here is a detailed explanation of why the correct answer is option 'A' (Assam):

1. Rajbongshi Population in Assam: The Rajbongshi community has a significant population in Assam, particularly in the western districts. They form one of the largest indigenous communities in the state. Their presence can be traced back to ancient times, and they have a strong cultural and historical connection to the region.

2. Geographical Distribution: The Rajbongshi people are predominantly found in the western districts of Assam, which share borders with neighboring states like West Bengal and Bihar. These districts include Kokrajhar, Bongaigaon, Dhubri, and Goalpara. These areas have a higher concentration of Rajbongshi population compared to other parts of Assam.

3. Cultural and Linguistic Identity: The Rajbongshi community has a distinct cultural and linguistic identity. They have their own language, Rajbongshi or Kamtapuri, which belongs to the Indo-Aryan language family. This language is widely spoken among the Rajbongshi people in Assam.

4. Historical Significance: The Rajbongshi community has played a significant role in the history of Assam. They have a rich history that dates back to the ancient Kamrupa kingdom. The Koch dynasty, which ruled parts of present-day Assam, was founded by a Rajbongshi ruler named Biswa Singha.

In conclusion, the Rajbongshi or Koch-Rajbongshi ethnic group primarily inhabits the state of Assam in India. They have a significant presence in the western districts of Assam and have a distinct cultural and linguistic identity. The Rajbongshi community has a rich history and plays an important role in the cultural fabric of Assam.