I believe the answer is:
C. existence of a large rural serf population
the serf population refers to the peasant whose existent is very similar with slaves ( they can be sold and bought) and were forced to work on the agricultural land owned by the nobles.
D. interdependence of lords and vassals
Lords is the one that own the land while vassals are the people who stay loyal under the lords' order in order to do their will. The vassals would be allowed to create a production on the land owned by the lords, but all the result is still belonged to the lords.