The two groups that serve lords are vassals and knights.
However, based on the options provided, the correct response is vassals, peasants. While peasants do not directly serve lords in the same way as vassals and knights, they do owe loyalty to the lords as part of the feudal system. So the most accurate response from your options is vassals, peasants.
If it had been an option, "knights" would also be correct in the context of serving lords, but since it's not, vassals are the primary group serving lords directly.