Answer: Option A is the correct answer: "an oath of loyalty and a pledge of military service."
Step-by-step explanation:
The feudal system that existed in medieval Europe was based on lords who owned property and vassals who serviced the lords. The vassal's responsibilities included an oath of loyalty and a commitment to military service to the lords. The oath of allegiance, derived from the Latin fidelitas, compelled the vassal to always stay faithful to the ruler.