Introduction:
Rajiv Gandhi, the 6th Prime Minister of India, was assassinated on May 21, 1991. His assassination was a tragic event that shook the nation and left a lasting impact on Indian politics.

Explanation:
The correct answer to the question is option 'C' - 1991. Here's a detailed explanation:

1. Background:
Rajiv Gandhi, the son of former Prime Minister Indira Gandhi, entered politics after her assassination in 1984. He served as the Prime Minister of India from 1984 to 1989.

2. Assassination:
On May 21, 1991, Rajiv Gandhi was assassinated while attending an election rally in Sriperumbudur, Tamil Nadu. The attack was carried out by a suicide bomber from the Liberation Tigers of Tamil Eelam (LTTE), a separatist militant organization from Sri Lanka.

3. Events leading to the Assassination:
The LTTE had been fighting for an independent Tamil state in Sri Lanka since the early 1980s. Rajiv Gandhi, during his tenure as Prime Minister, had sent the Indian Peacekeeping Force (IPKF) to Sri Lanka to intervene in the conflict. However, the IPKF faced opposition from both the LTTE and other factions in Sri Lanka, leading to a complex and challenging situation.

4. Motive behind the Assassination:
The LTTE perceived Rajiv Gandhi's intervention in Sri Lanka as a threat to their cause. They believed that his assassination would not only eliminate a powerful opponent but also draw international attention to their struggle for an independent Tamil state.

5. Immediate Aftermath:
The assassination of Rajiv Gandhi sent shockwaves across India and the world. The nation mourned his loss, and there was a massive outpouring of grief and anger. The LTTE was widely condemned for the act, and there were calls for justice and action against the organization.

6. Investigation and Trials:
Following the assassination, extensive investigations were carried out to identify and apprehend the perpetrators. Several individuals, including LTTE members, were arrested and put on trial. In 1998, the trial concluded, and four individuals, including the suicide bomber, were convicted and sentenced to death.

Conclusion:
In conclusion, Prime Minister Rajiv Gandhi was assassinated in the year 1991. His assassination was a significant event in Indian history, marking a tragic loss for the nation and leaving a lasting impact on its political landscape.