Which of the following is a major hallmark of the period of middle childhood?

Piaget's theory of cognitive development divides childhood into stages by age: the sensorimotor stage (birth to 2 years, action-based learning), the preoperational stage (roughly 2 to 6 years, symbolic and make-believe play but not yet logical thought), the concrete-operational stage (roughly 7 to 11 years, logical reasoning applied to concrete, tangible objects and situations), and the formal-operational stage (roughly 11 years onward, abstract and hypothetical reasoning).

Middle childhood spans roughly ages 6 to 12, overlapping substantially with the concrete-operational stage. During this period children become able to reason logically about real, tangible problems -- classifying objects, ordering them by size, and understanding that a quantity stays the same even when its appearance changes -- while still finding purely abstract or hypothetical reasoning difficult.

• Emergence of make-believe play belongs to the earlier preoperational stage (about 2 to 6 years), before middle childhood begins.

• Rapid motor-skill gains and physical growth spurts are most marked in infancy and early childhood; physical growth during middle childhood is comparatively steady rather than rapid.

• Scientific reasoning and abstract thought are hallmarks of the later formal-operational stage of adolescence, not of middle childhood.

Because middle childhood is defined by this shift to logical, concrete-operational thought, the development of logical thought that is concrete in nature is the major hallmark of this period.