The power of the church was most weakened by what?
a. population growth
b. the crusades
c. the reformation
d. the rise of nations

The church here, whose power was weakened is the catholic Church.

It was weakened by c. the reformation, that is the Protestant Reformation that tool place in Europe in 16th Century. The Revolution was a split in which huge populations of western Europe left the Catholic Church and established their own Churches.