Final answer:
In the feudal system, a vassal demonstrated loyalty to a lord primarily by providing military service in exchange for land and protection.
Step-by-step explanation:
A vassal is expected to show loyalty to a lord by providing military service. In medieval Europe's feudal society, a feudal system was established based on a series of reciprocal legal and military obligations among the warrior nobility, which revolved around the three key concepts of lords, vassals, and fiefs.
Vassals were granted pieces of land, known as fiefs, in exchange for their service, which was chiefly military in nature but could also include advising the lord and attending his court. Outside of military service, serfs or peasants worked the land to produce goods and provide labor for the lord, essentially supporting this feudal society.