Explanation:

- Hindi Diwas is celebrated to mark the adoption of Hindi as one of the official languages of India in 1949.

- Option 4: 14 September

- This is the correct answer. Hindi Diwas is observed annually on 14 September.

- Option 1: 2 October

- This is not correct. 2 October is observed as Gandhi Jayanti, not Hindi Diwas.

- Option 2: 14 March

- This date is not linked to the commemoration of Hindi Diwas.

- Option 3: 6 April

- This date does not relate to Hindi Diwas either.