Which of the following days was observed on 21st May 2025?

The correct option is National Anti-terrorism day.

Here is the step-by-step explanation of why this option is correct:
National Anti-Terrorism Day is observed every year in India on May 21. Therefore, on 21st May 2025, National Anti-Terrorism Day was observed.

Significance of the Day:
This day marks the death anniversary of India's former Prime Minister, Rajiv Gandhi, who was assassinated on May 21, 1991, during an election rally in Sriperumbudur, Tamil Nadu. The day is observed to generate awareness about the danger of terrorism and violence, and its effect on people, society, and the nation as a whole.

Let's look at the other options for clarity:
1. National Human Rights Day (Human Rights Day) is celebrated internationally on December 10 every year.
2. National Voters Day is celebrated in India on January 25 to mark the foundation day of the Election Commission of India.
3. Indian Navy Day is celebrated on December 4 to recognize the role and achievements of the Indian Navy, particularly during Operation Trident in the Indo-Pakistan War of 1971.