Final answer:
The 'Boy ar Tsar' refers to Ivan IV, also known as Ivan the Terrible. He was the first person to declare himself Tsar of all the Russias and expanded the Russian Empire during his rule.
Step-by-step explanation:
The 'Boy ar Tsar' refers to Ivan IV, also known as Ivan the Terrible. Ivan IV was the first person to declare himself Tsar of all the Russias, a title which means 'Caesar' in Russian. He was known for expanding the Russian Empire and annexing various territories. Ivan IV ruled from 1547 to 1584, and his reign had a significant impact on Russian history.