Kievan Rus is the first East Slavic state that emerged during the 9th century when the Viking Prince Oleg began uniting the East Slavic lands. This state flourished itself with an unstable political system until the 13th century.
Towards the end of the 9th century, the state under Vladimir the Great began introducing Christianity to the tribes, thus heralding the Christian faith in the lands. The federation of states, which is now known as Kievan Rus, gained its zenith during Yaroslav in the early 11th century.
The correct answer for the branch of Slavic tribes that would eventually be the Kievan Rus is the Eastern tribe
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