Explanation:

- The Constituent Assembly of undivided India first convened to draft the Indian constitution.

- Option 1: 6th December, 1946

- This date is incorrect for the first meeting.

- Option 2: 9th December, 1946

- Correct Answer. This was the date when the Constituent Assembly first met.

- Option 3: 20th February, 1947

- This date is related to the announcement of the British withdrawal plan, not the Assembly's first meeting.

- Option 4: 3rd June, 1947

- This date marks the presentation of the plan for partition, not the Assembly's first meeting.