The concept of Developmental Task was proposed by

A developmental task is a task that arises at or about a particular period in a person's life, the successful achievement of which leads to satisfaction and success with later tasks, while failure leads to unhappiness, social disapproval, and difficulty with the tasks that follow. This framework organises human growth into a sequence of age-linked tasks running from infancy through old age.

Robert J. Havighurst introduced and systematised this concept in his 1948 book "Developmental Tasks and Education", laying out specific tasks across six life stages (infancy and early childhood, middle childhood, adolescence, early adulthood, middle age, and later maturity). Since Havighurst is the theorist who coined and built this framework, Havighurst is the answer.

Each other option names a different theorist associated with a different framework:

• Hollingworth is known for pioneering research on adolescent psychology and on gifted and exceptional children, not for the developmental-task framework.

• Jean Piaget's stage theory describes changes in cognitive structures (schemas) across the sensorimotor, preoperational, concrete operational, and formal operational stages -- a framework of cognitive development, not of developmental tasks.

• G. Stanley Hall is known for the recapitulation theory and for describing adolescence as a period of "storm and stress", not for the developmental-task framework.