The correct answer is option C) Lahore Session of the INC.

Explanation:

The formal partition of India was marked by the Lahore Session of the Indian National Congress (INC) held in 1929. This session is also known as the "Lahore Resolution" or the "Lahore Congress".

Key Points:

1. Demand for Purna Swaraj:
- The Lahore Session of the INC was significant because it was during this session that the demand for complete independence or Purna Swaraj was made.
- The resolution for Purna Swaraj was passed on December 31, 1929, and it declared that January 26, 1930, would be celebrated as Independence Day.

2. Civil Disobedience Movement:
- The Lahore Session and the declaration of Purna Swaraj laid the foundation for the Civil Disobedience Movement launched by Mahatma Gandhi in 1930.
- The movement aimed to challenge British authority and gain independence through non-cooperation, protests, and acts of civil disobedience.

3. Impact on Partition:
- The demand for Purna Swaraj and the subsequent Civil Disobedience Movement led to increased tensions between the Indian National Congress and the Muslim League.
- The Muslim League, fearing the dominance of Hindus in a unified India, began demanding a separate nation for Muslims, which eventually led to the partition of India and the creation of Pakistan in 1947.

4. Historical Significance:
- The Lahore Session of the INC marked a turning point in the Indian independence movement as it shifted the focus from constitutional reforms to complete independence.
- It provided a platform for nationalists to unite and demand self-rule, leading to a more radical approach in the struggle against British colonial rule.

In conclusion, the Lahore Session of the INC in 1929, where the demand for Purna Swaraj was made, marked the formal partition of India as it set the stage for the subsequent Civil Disobedience Movement and the demand for a separate nation by the Muslim League.