Which Child Does Piaget's Theory Have the Most Difficulty Explaining

Which child does Piaget's theory have the most difficulty explaining?

A) Ivy, who has mastered the conservation and three-mountain tasks
B) Rose, who fails both the conservation and three-mountain tasks
C) Daisy, who has mastered the conservation task but fails the three-mountain task
D) Flora, who not only has mastered the conservation and three-mountain tasks but can explain the intricacies of these tasks to others