The correct answer is option 'C' - Chauhans.

The Chauhans were a dynasty that played a significant role in the history of Delhi. They overthrew the Tomaras, who founded Dhillika (the ancient name for Delhi) in 736 A.D., and occupied Delhi in the twelfth century.

Here is a detailed explanation of the Chauhan dynasty's rise to power and their occupation of Delhi:

1. Introduction to the Tomaras and Dhillika:
- The Tomaras were an ancient Indian dynasty that ruled over the region of Delhi from the 8th to the 12th century.
- They founded the city of Dhillika in 736 A.D., which later came to be known as Delhi.
- The Tomaras initially enjoyed a significant amount of power and influence, but their rule weakened over time.

2. Rise of the Chauhans:
- The Chauhans were a Rajput clan that originated from the Chauhan kingdom in Rajasthan.
- They were known for their bravery, martial skills, and strong military capabilities.
- The Chauhans gradually expanded their territory and influence in northern India.

3. Chauhans overthrowing the Tomaras:
- The Chauhans saw an opportunity to seize power in Delhi when the Tomara dynasty weakened.
- Prithviraja Chauhan, the most famous ruler of the Chauhan dynasty, defeated the Tomaras and captured Delhi in the twelfth century.
- Prithviraja Chauhan's victory in Delhi marked the beginning of the Chauhans' stronghold in the region.

4. Occupation of Delhi by the Chauhans:
- After capturing Delhi, the Chauhans established their stronghold in the city.
- They expanded their kingdom and ruled over a vast territory that included present-day Delhi, parts of Rajasthan, Haryana, and Uttar Pradesh.
- The Chauhans made Delhi their capital and developed it as an important center of governance, trade, and culture.

In conclusion, the Chauhans overthrew the Tomaras, who founded Dhillika in 736 A.D., and occupied Delhi in the twelfth century. This marked the rise of the Chauhan dynasty and their significant influence in Delhi's history.