The true statements are: A. Knights are able-bodied men who are hired by the vassal for the protection of the lord and C. Feudal aid is where vassals would be ready to assist monetarily such as in giving ransom money in time of need. Knights were, in effect, mercenaries with a specific code of honor that would bind them to the lords. They would get titles and castles in exchange for their military service. The C statement is true because vassals were bound to assist their lords both financially and in other regards. B is not true because vassals didn't own the land, even though they controlled it - the whole land was owned by the monarch. The D statement is just nonsensical - knights didn't serve serfs in any way.
