What are two main factors that made the Catholic Church the most powerful institution in western Europe during the Middle Ages?

The two main factors are:

The fact that all of Europe was catholic at a time, until the eastern division of the church and the birth of the Eastern Orthodox Church.

And the fact that the Catholic church enjoyed legitimacy as inheritors of the Roman Empire, at least when it comes to religous power. This gave the Catholic Church acceptance among the many governments in Europe.

your answer will most likely be a.