D. The popes, missionaries, and monks.
Step-by-step explanation:
Popes, missionaries, and monks specialized in spreading religion, primarily Christianity throughout Europe.
The first choice would be incorrect because knights were fighters, they did not spread religion.
The second choice would be incorrect because Saint Patrick and Saint Benedict were not really known for spreading Christianity around Europe.
The third choice would be incorrect for also the reason above, along with this, Charlemagne was a king, not someone who spread religion.