Final answer:
The 'Enlightened Tsar' was Alexander II, notably for his progressive reforms including the emancipation of the serfs in 1861, but his reign ended with his assassination by a radical group.
Step-by-step explanation:
The Russian monarch who was nicknamed 'the Enlightened Tsar' is Alexander II. He earned this title due to his progressive policies, notably his moves toward legal reform and most significantly, the emancipation of the serfs in 1861, which was intended to modernize Russia and alleviate some of its backwardness. Despite his efforts to reform, Alexander II's life was ended by an assassination executed by The People's Will, a radical terrorist group that was influenced by anarchist socialist ideas hoping that his death would incite a revolution.