Which describes the role of a vassal? to give the king a fiefdom, to work the land, to pay homage to a knight, to be ready to serve in a standing army, to function as liege-lord to knights

The role of a vassal was to pay homage to a knight.

Homage was a special ceremony of allegiance. A vassal was in a subordinate position, pledging his loyalty and homage to the knight he served. A vassal could give "fealty" -- a lesser oath of loyalty -- to more than one lord. But "homage" was a personal relationship of loyalty to one specific "liege-lord." The knight or lord would be the "liege-lord" of the vassal.

Feudalism was a complex system of relationships between people of different classes. The king stood at the top of the complex system, and nobles were viewed as holding their lands in exchange  for their military service to the king.  The nobles then had vassals underneath them.  And at the bottom of the system, depending on the upper classes for protection and laws, were the peasants, who gave a share of their produce to the lords above them as their part of the exchange.

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