The correct answer is c. The decree weakened the power of European monarchs.
The decree, known as the Dictatus Papae, asserted the authority of the papacy over secular rulers in matters of church appointments and governance. This introduced a significant power struggle between the church and monarchs, ultimately leading to diminished influence of monarchs in religious appointments and governance. This struggle is famously illustrated by the Investiture Controversy, where the authority of the pope was emphasized over that of kings and emperors.