Explanation:

- Mangal Pandey was a key figure in the Indian Rebellion of 1857, known for attacking British officers at Barrackpore.

- Option 1 (21 March 1857): Not correct; nothing significant happened on this date involving his execution.

- Option 2 (23 March 1857): Also incorrect; not the date of his execution.

- Option 3 (31 March 1857): Incorrect; again not the execution date.

- Option 4 (29 March 1857): This is the date when Pandey attacked British officers, but he was not hanged on this date. His execution actually took place on 8 April 1857.

- So, none of the provided options are accurate for his hanging date; however, option 4 is about the date of his attack, which is often confused with his execution.