which of the following best describes serfs?

an agricultural laborer bound under the feudal system to work on his lord's estate.

Serfs are often described as unfree. More accurately, because that term is usually misunderstood, they might best be called peasants who were bound to the land they lived on but did not own.  

The problem with the term unfree, is that most people think it means slave, which it was not. Serfs did not have a right to move away from the land they lived on, but unlike slaves they could not be bought or sold, and unlike slaves, they had a right to live and work on the land. They owed the lord rent, usually in the form of labor or a part of the crop, but they had a right to expect the lord to protect them.

it would be a. that is the only one i could think of