What was one significant indication that the european feudal system was effective?

The European feudal system was effective because it keep the people safe from marauding bandits. I don't believe that the system was fair. It infringed on the human rights of the serfs, but I suppose in the context of war and life or death, it was the better option. The Feudal system was structured so that the king of the country owned all the land. Some of the land was given to the church and the rest was rented out to the Barons. They in turn, gave land to the Knights in return for military service and protection. The lowest class of people in this structure was the serfs. They were allowed to live on the land by the Knights in exchange for food, service and free labour. 

Ibelieve the answer is c