Answer : 3. "22 दिसंबर "
Explanation :

National Mathematics Day is celebrated every year on 22 December to commemorate the birth anniversary of the great mathematician Srinivas Ramanujan. In the year 2012, the then Prime Minister Manmohan Singh had declared 22 December as National Mathematics Day. In 1991, Srinivas Ramanujan's biography 'The Man Who Knew Infinity' was published. Ramanujan was born on this day in 1887 in a Brahmin Iyengar family in Erode, Tamil Nadu.