The correct answer is option 'B' - Aravalli.

The Aravalli Range, also known as Aravalli Hills, is the oldest mountain range in India. It is a range of fold mountains that stretches across the states of Rajasthan, Haryana, and Gujarat. The Aravalli Range runs approximately 692 kilometers from southwest to northeast.

Here's a detailed explanation of why the Aravalli Range is considered the oldest mountain range in India:

1. Geological Age:
- The Aravalli Range is estimated to be around 350 million years old, making it one of the oldest mountain ranges in the world.
- It dates back to the Proterozoic era, specifically the Precambrian period, which lasted from about 4.6 billion to 541 million years ago.

2. Formation:
- The Aravalli Range was formed during the collision between the Indian Plate and the Eurasian Plate.
- This collision resulted in the folding and uplifting of the Earth's crust, leading to the formation of the Aravalli Range and other mountain ranges in the region.
- The intense tectonic activity during this period caused the rocks in the Aravalli Range to undergo immense pressure and folding.

3. Erosion and Weathering:
- Over millions of years, the Aravalli Range has undergone extensive erosion and weathering.
- The once towering peaks have been significantly worn down, giving the range a relatively low and rounded appearance.
- The erosion has been mainly caused by the action of rivers, wind, and other natural processes.

4. Significance:
- The Aravalli Range has great geological, ecological, and cultural significance.
- It serves as a watershed for many rivers, including the Banas, Luni, and Sabarmati rivers.
- The range is home to a diverse range of flora and fauna, including several endangered species.
- It also holds historical and cultural importance, with ancient forts, temples, and archaeological sites located within its vicinity.

In conclusion, the Aravalli Range is considered the oldest mountain range in India due to its geological age, formation during the Proterozoic era, and extensive erosion over millions of years.