Final answer:
In medieval Europe's feudal society, the knights were the military class who received fiefs in exchange for protection and military service. Correct option is c)
Step-by-step explanation:
The military class in medieval Europe's feudal society was the knights. They received land, or fiefs, in exchange for protection and military service to the lords. The feudal system was a mutually supportive system where the lords owned the land, and the knights provided the necessary military support to maintain the lords' land and enforce their rule. Serfs worked the land and were tied to it, providing labor in return for protection offered by the lord's castle or the walls of his city. Overall, this social structure formed the basis of medieval European society.