Final answer:
One of the most exciting achievements in developmental psychology in the past century is Jean Piaget's research on cognitive development in children.
Step-by-step explanation:
One of the most exciting achievements in developmental psychology in the past century is the research conducted by Jean Piaget on cognitive development in children. Piaget's work revolutionized our understanding of how children think and learn, particularly in the areas of object permanence and logical reasoning. For example, Piaget's findings showed that very young children do not demonstrate object permanence, which refers to the understanding that physical things continue to exist, even if they are hidden from us. This research has had a significant impact on educational practices and parenting strategies.