Answer:
king, clergy, knights, serf.
Explanation:
The king would be the head, therefore leading the social order under feudalism. The Clergy would be second on the social order, as they are under the church therefore closed to the kings social standing.
The Knights would be next as they have about the same standing as the Serf.
A serf is just a person who works under the king, tending to his courtyard and such things.
