Explanation:

- National Hindi Diwas is a significant day in India, observed to promote and celebrate the Hindi language.

- It commemorates the adoption of Hindi as one of the official languages of India on September 14, 1949.

- The day is marked with events in schools, colleges, and other institutions to honor Hindi literature and language.

- The aim is to encourage people to embrace and use Hindi more actively in their daily lives.

Option 1: 14th September

- Hindi Diwas is observed on this date every year.

Option 2: 16th September

- No significant association with Hindi Diwas.

Option 3: 18th September

- No substantial link to Hindi Diwas events.

Option 4: 20th September

- It is not recognized as Hindi Diwas.