Final answer:
The adoption of Eastern Orthodox Christianity in Kyivan Rus' history led to the development of rulers taking the title of czar, the use of the Cyrillic alphabet, and the development of trade with the Byzantine Empire.
Step-by-step explanation:
The development in Kyivan Rus' history that led to the other three developments mentioned in the question is the adoption of Eastern Orthodox Christianity.
When Prince Vladimir I of Kyiv allied with the Byzantine emperor Basil II in 987, he converted from paganism to Christianity. This led to the mass baptism of his subjects in the Dnieper River and the subsequent adoption of the Byzantine Orthodox Christian faith by Kievan Rus'.
Once Kievan Rus' adopted Orthodox Christianity, it influenced other aspects of their culture and society. The rulers took the title of czar, the Cyrillic alphabet became the basis of their written language, and trade developed along the rivers, linking them to the Byzantine Empire. These developments were interconnected and stemmed from the initial adoption of Eastern Orthodox Christianity.