The hallmark cognitive achievement of the preoperational stage is:

Concept

Cognitive-developmental stages are distinguished by newly available ways of representing and transforming experience. In the preoperational stage (about ages 2–7), children can use symbols—language, images, and pretend objects—but do not yet reliably perform reversible logical operations.

Application

The stem asks for the defining cognitive gain of this stage. The new capacity to let one thing stand for another directly identifies symbolic representation.

Contrast

• Object permanence develops during the sensorimotor stage and concerns the continued existence of unseen objects.

• Mental operations become reliable in the concrete operational stage, when children can use reversible logic on concrete situations.

• Formal reasoning characterizes the formal operational stage, where abstract and hypothetical reasoning becomes possible.

Result

Therefore, the hallmark cognitive achievement of the preoperational stage is symbolic representation.