Which day is celebrated as International Mother Language Day?

The correct answer is February 21.

Explanation:
International Mother Language Day is celebrated globally on February 21 every year. This day is observed to promote awareness of linguistic and cultural diversity and to encourage multilingualism.

The initiative was first announced by the United Nations Educational, Scientific and Cultural Organization (UNESCO) on November 17, 1999, and was formally recognized by the United Nations General Assembly in 2002. February 21 was specifically chosen to commemorate the 1952 Bengali Language Movement in Bangladesh, where students and activists campaigned for the recognition of Bengali as one of the state languages of Pakistan.