Which Session of Congress and Muslim League reached an understanding of creating a joint front against the British regime?

The correct option is Lucknow.

To understand how the Indian National Congress and the All-India Muslim League reached an agreement to present a joint front against the British regime, we can examine the historical context step-by-step:

1. Historical Context:
Before 1916, the Indian National Congress (dominated by moderate and extremist nationalist leaders) and the All-India Muslim League (formed in 1906 to protect Muslim interests) generally functioned on different political platforms with varying demands.

2. The Lucknow Session of 1916:
In December 1916, both the Indian National Congress and the All-India Muslim League held their annual sessions simultaneously in Lucknow, Uttar Pradesh. This joint meeting was a landmark event in the Indian national movement.

3. The Lucknow Pact:
During this session, both organizations signed a historic agreement known as the Lucknow Pact. Under this agreement, the Congress agreed to separate electorates for Muslims in provincial legislatures, and both parties jointly demanded self-government (Home Rule) from the British regime. This established a unified, joint nationalist front against British colonial rule.