In Europe's feudal system which group was tied to the land they worked unable to leave without permission?

Serfs.

Explanation:

They occupied lands that belonged to their lords and they weren't able to leave without permission. This community was highly rewarded in Crusades, some of them achieve their freedom, other just died, but some acquired big loots as a result of the savvy engagements.

there are 4 options

either it could be barons or vassals or serfs or i knights

it is serfs

the answer is serfs