Jawaharlal Nehru unfurled the tri-colour national flag on the banks of the Ravi as the clock struck midnight on 31st December 1929.

Option 1: 31st December 1929 - This is the correct answer as stated in the question. Nehru celebrated the New Year`s Eve of 1929 by unfurling the national flag on the banks of the Ravi.

Option 2: 26th January 1930 - This date is significant as it marks the declaration of India`s independence by the Indian National Congress, through the adoption of the Purna Swaraj (complete independence) resolution. However, this date is not the specific occasion mentioned in the question.

Option 3: 31st December 1931 - This date does not correspond to the specific event mentioned in the question. It is important to pay attention to the year mentioned in the question as it has a specific historical context.

Option 4: 26th January 1933 - This date does not correspond to the specific event mentioned in the question. January 26th became significant in the later years as it is the date when the Indian Constitution was adopted in 1950.

In summary, the correct answer