Step 1: Know the event origin
International Mother Language Day was declared by UNESCO in 1999 and has been observed every year since 2000 to promote linguistic and cultural diversity.
Step 2: Historical background
The date — 21st February — commemorates the 1952 Bengali Language Movement in Dhaka (now in Bangladesh), where students died while protesting for recognition of their language.
Step 3: Eliminate wrong choices
12th, 2nd, and 22nd February have no relation to this event.
Thus, the correct answer is 21st February.