C is correct answer because this Concordat was a result of a struggle between Popes and Holy Roman Emperors during 11th and 12th Century. Namely, this was a large dispute over power, which had large consequences to political tides in Europe. At the end agreement was made according to which Pope is choosing bishops, but they are swearing oath to the ruler.
That is why A and B are not correct, as the conflict was political between Pope and Emperor.
