Answer:
D) the Roman Catholic Church
Step-by-step explanation:
Roman Catholic Church was the most centralized institution during the Middle Ages. The influence of the church was so great that Popes were even able to overthrow the rulers in certain countries.
As religion was a central part of everyday's life of people it was impossible to imagine any kind of social interaction between the people without the approval of the church. It was the only real institution that survived the period of Ancient Rome and had adapted to new political order.