Answer : 4. "12 January"
Explanation :

National Youth Day is celebrated all over India on 12 January every year. This day is celebrated on the occasion of the birth anniversary of Swami Vivekananda. The Government of India officially declared National Youth Day in the year 1984 and it is being celebrated across the country since 1985. Swami Vivekananda's real name was Narendranath Dutt. In 1898 he established Ramakrishna Math in Belur.