Concept:
- When a question adds a region to a year, it is warning that more than one answer exists for that year.
- Listing both sessions of 1920 and then applying the region settles it safely.
Step 1: List every Congress session of 1920.
There were two. A special session was held in September and the regular annual session in December. Any answer must be one of these.
Step 2: Locate each on the map.
The special session of September 1920 met at Calcutta in Bengal, which is eastern India. The annual session of December 1920 met at Nagpur, which is in central India.
Step 3: Apply the clue given.
Only Calcutta lies in eastern India, so that is the session being asked about.
Step 4: Confirm from what happened at it.
It was at Calcutta that Gandhiji persuaded the other leaders to adopt a Non-Cooperation programme in support of Khilafat and for swaraj, a decision later confirmed at Nagpur.
The importance of the session matches the importance of the question.
Final Answer: The 1920 session in eastern India was held at Calcutta, now Kolkata.