The correct answer is option 3 - Surat. The Indian National Congress, which was formed in 1885, had a diverse range of members with different political ideologies. In 1907, during the Surat session of the Congress, a split occurred between the Moderates and the Extremists.

1. Option 1 - Nagpur: Nagpur session of 1908 was significant for the Congress as it marked the formal end of the split between Moderates and Extremists. So, this option is incorrect.

2. Option 2 - Allahabad: The Congress held its sessions in Allahabad in 1888, 1892, and 1902. However, the split between the Moderates and Extremists did not occur in any of these sessions. Hence, this option is incorrect.

3. Option 3 - Surat: The Surat session of 1907 was a crucial turning point for the Indian National Congress. Differences between the Moderates, who believed in peaceful and constitutional methods, and the Extremists, who advocated for more radical and revolutionary approaches, became apparent during this session. Thus, this is the correct option.

4. Option 4 - Calcutta: The Congress held multiple sessions in Calcutta