Final answer:
Erik Erikson proposed three adulthood stages of psychosocial development, marked by tasks or crises that must be resolved.
Step-by-step explanation:
The theorist who proposed three adulthood stages of psychosocial development, each of which was marked by a given task or crisis that must be resolved is Erik Erikson. According to Erikson's theory of psychosocial development, there are eight stages of development that individuals go through from infancy to late adulthood. In each stage, there is a specific task or conflict that needs to be resolved for healthy personality development. Successful completion of these tasks results in a sense of competence and a healthy sense of self.