farmland for crops

giving part of crops produced to manor
provide knights to defend this land in exchange for loyalty and military service

this list is describing

Vassals

fief

viking settlements

serfs

The list is describing the feudal system, where vassals are granted land (fief) by a lord or monarch in exchange for their loyalty and military service. The vassals in turn provide part of their crops to the lord and also supply knights to defend the land. Serfs may also be part of the system, as they were peasants who worked on the land and were obligated to provide labor and a portion of their produce to the lord. Viking settlements refers to the specific historical context wherein Vikings established their own settlements across Europe during the Middle Ages.