How could serfs move up from the bottom of the social hierarchy?
a. they could prove themselves in battle.
b. they had to serve as a serf for a certain number of years.
c. they had to be freed by their lord.
d. they could marry into a higher level of the hierarchy.

C. They had to be freed by their lord.

Explanation:

Servitude (from the Latin servus) was a form of social and juridical contract typical of feudalism by which one person - the servant, usually a peasant - is left to serve and subject to the lordship of another - the feudal lord, usually a noble or a high ecclesiastical dignitary, or even an institution such as a monastery. During the Middle Ages, a servant was a person who served in conditions close to slavery. The main difference with respect to a slave was that, in general, he could not be sold or separated from the land he worked in and that he was legally a "free man." The feudal lord had the power to decide on numerous matters of the life of his servants and on his possessions. The servant could not betray the feudal lord, since he provided him with housing, part of the crops and his clothes.

Characteristic of the servitude of a servant was the set of obligations consigned, such as the inability of the servant to acquire or sell real estate, the submission to the political, judicial and fiscal authority of the feudal lord, the obligation to provide military services to his lord. and the delivery of part of their work or product. The condition of a servant was hereditary and he could not leave his land without the permission of his lord. Normally when serfdom predominated, land alone could not be sold, because it was associated with political powers. Instead, the land could be transferred through wars or betrothal.

i would go with the last one