in what role did both nobles and knights serve?\no vassal\no serf\no monarch\no priest

Brief Explanations:

In feudal - era Europe, both nobles and knights were vassals. They held land from a lord in return for military service and other obligations. Serfs were peasants bound to the land. Monarchs were the rulers at the top of the feudal hierarchy. Priests were part of the religious establishment.

Answer:

A. vassal