Final answer:
Prince Vladimir I of Kiev was the Russian ruler who adopted the Eastern Orthodox Church as the state religion through an alliance with the Byzantine Empire and by converting from paganism to Christianity in 987.
Step-by-step explanation:
The Russian ruler who adopted the Eastern Orthodox Church as the official religion of the Russian state was Prince Vladimir I of Kiev. In the year 987, Prince Vladimir formed an alliance with the Byzantine emperor, Basil II, and converted from paganism to Christianity.
This monumental event included the marriage to Basil II's sister, Anna, and his subjects' mass baptism in the Dnieper River. Vladimir's conversion played a critical role in shaping the cultural and religious identity of Russia, which began incorporating Byzantine art and architecture.