During the Middle Ages, the group that held the majority of the power varied by region and time period. However, in general, the answer would be A. Nobles, as they were often the ones who held feudal titles and controlled the land, wealth, and military resources.
The clergy (B) also held significant power and influence, particularly in matters of religion and education. Knights (C) were a part of the noble class and had military prowess but did not hold as much political power as the nobles. Kings (D) could be powerful figures, but their power was often limited by the nobles who held land and resources.