Which of the Following Was the Most Important Political and Social

Which of the following was the most important political and social change after the collapse of Charlemagne's empire?

A) While monasteries became more independent, their political power decreased.
B) The popes claimed more political authority to replace the vacuum left after Charlemagne's death.
C) The class of knights subjugated previously free peasants, leading to serfdom in much of western Europe.
D) Merchants became wealthier with the revival of long-distance trade, and demanded more political power.