The correct answer is D. The oath of loyalty between vassal and lord.
Step-by-step explanation
Feudalism was a social, political and economic system that prevailed during the Middle Ages, from the ninth to the fifteenth century based on a strong monarchical system and a system of loyalty. The feudal system was characterized by the decentralization of power, exercised through nobles organized in feuds. The word feudalism derives from the word fief that was a contract between two people. In this way, the vassal received land to inhabit and military protection from the feudal lord. In exchange for this, the vassal should work on the land received and defend the lord if necessary due to both the fief sign and the loyalty relationship between both. So, the correct answer is D. The oath loyalty between vassal and lord.