The correct answer is "protection".
In feudal society, a vassal was a person invested with a fief for which they had to povide services to a lord. First, a feudal contract was made, and under that contract, the lord was bound to provide the vassal with the fief, i.e. to provide protection and to do him justice in court. In return the Lord could demand the services which were attached to the fief, and these could include all kinds of services - military, administrative etc.