Concept:May 21 is officially observed in India as National Anti-Terrorism Day to commemorate the death of former Prime Minister Rajiv Gandhi.Explanation:The date was chosen because Rajiv Gandhi was assassinated on May 21, 1991.On this day, universities and institutions hold debates, discussions, and pledge ceremonies to spread awareness.The aim is to reduce terrorism and the radicalization of youth, which harms national interest.Answer:National Anti-Terrorism Day (Option D).