Correct Answer: UNESCO
Explanation: UNESCO (United Nations Educational, Scientific and Cultural Organization) declared 21st February as International Mother Language Day to promote linguistic diversity and multilingualism. This day is particularly significant for Bangladesh, as it commemorates the Language Movement of 1952.

Incorrect Option 1: UNICEF
Explanation: UNICEF focuses on children's welfare and does not handle matters related to language days.

Incorrect Option 2: IMF
Explanation: The IMF (International Monetary Fund) deals with economic and financial issues, not cultural or linguistic matters.

Incorrect Option 3: UNDP
Explanation: The UNDP (United Nations Development Programme) focuses on global development, but it is not related to International Mother Language Day.

Incorrect Option 4: UNHCR
Explanation: UNHCR (United Nations High Commissioner for Refugees) deals with refugee issues and is not involved with language promotion.