The correct answer is option 'C' - Purvanchal.

Purvanchal is a collective term used to refer to the mountain ranges in the eastern part of India that form its boundary with Myanmar. It is also known as the Eastern Ghats or the Purvanchal Range. Let's explore these mountain ranges in more detail:

1. Location:
- The Purvanchal range is located in the eastern part of India, spanning across several states including Arunachal Pradesh, Nagaland, Manipur, Mizoram, and Tripura.
- It forms the eastern boundary of India with Myanmar (formerly known as Burma).

2. Eastern Ghats:
- The Eastern Ghats are a discontinuous range of mountains that run parallel to the Bay of Bengal.
- These mountains are older and lower in elevation compared to the Western Ghats, which are located on the western coast of India.
- The Eastern Ghats are not as well-known as the Western Ghats, but they are still significant in terms of their ecological and geographical importance.

3. Physiography:
- The Purvanchal range is characterized by rolling hills, plateaus, and valleys.
- It is a part of the larger Himalayan mountain system and is considered an extension of the Arakan Yoma range in Myanmar.
- The mountain range is home to several rivers, including the Brahmaputra, Barak, Manipur, and Chindwin, which flow through the region.

4. Biodiversity:
- The Purvanchal range is known for its rich biodiversity, with dense forests and numerous wildlife species.
- It is home to several national parks and wildlife sanctuaries, including Namdapha National Park in Arunachal Pradesh and Dampa Tiger Reserve in Mizoram.
- The region supports a wide range of flora and fauna, including rare and endemic species.

5. Cultural Significance:
- The Purvanchal range is inhabited by various indigenous communities, each with its distinct culture, language, and traditions.
- The region has a rich cultural heritage, with vibrant festivals, traditional arts, and crafts.
- The diverse ethnic groups living in the region contribute to the cultural mosaic of eastern India.

In conclusion, the mountain ranges in the eastern part of India forming its boundary with Myanmar are collectively called Purvanchal or the Eastern Ghats. These ranges are significant in terms of their geographical features, biodiversity, and cultural heritage.