Consider the following statements.
1. 21st February is declared to be the International Mother Language Day by UNICEF.
2. The demand that Bangla has to be one of the national languages was raised in the Constituent Assembly of Pakistan.
Which of the above statements is/are correct?

The correct option is 2 only.

Step-by-step breakdown and explanation:

Statement 1: "21st February is declared to be the International Mother Language Day by UNICEF."
This statement is incorrect.
International Mother Language Day was proclaimed by the General Conference of the United Nations Educational, Scientific and Cultural Organization (UNESCO) in November 1999, not UNICEF. It has been observed globally every year since February 2000 to promote linguistic and cultural diversity and multilingualism. The United Nations General Assembly formally welcomed the declaration of the day in its resolution of 2002.

Statement 2: "The demand that Bangla has to be one of the national languages was raised in the Constituent Assembly of Pakistan."
This statement is correct.
On February 23, 1948, Dhirendranath Datta, a member of the Constituent Assembly of Pakistan from East Bengal, moved an amendment proposing that Bengali (Bangla) be made one of the official languages of the Assembly alongside Urdu and English. This demand was raised because Bengali was spoken by the majority of Pakistan's population at the time. The rejection of this proposal sparked widespread protests, laying the foundation for the Bengali Language Movement in East Pakistan.

Conclusion:
Since Statement 1 is incorrect and Statement 2 is correct, the right answer is 2 only.