Charlemagne was able to

a.) convert his subjects to islam
b.) reunite the eastern and western christian churches
c.) avoid war with his neighbors
d.) create a strong and stable government

Charlemagne was able to reunite the Eastern and Western Christian Churches. The correct option among all the options that are given in the question is the second option or option "B". I hope that this is the answer that you were looking for and it has come to your help.