In which stage of Piaget's Cognitive Development theory does the ability of…

Piaget's theory of cognitive development describes four sequential, qualitatively distinct stages of mental growth — sensorimotor, pre-operational, concrete-operational, and formal-operational — each stage adding a new kind of thinking that was absent in the one before it.

Abstract thinking and generalization — reasoning about hypothetical and symbolic ideas that are not tied to any physical object, and drawing general logical principles from specific instances — is the defining new capability of the formal-operational stage, which emerges from around 11-12 years of age onward.

• Sensory motor stage: the infant only coordinates reflexes and simple sensorimotor actions (such as developing object permanence); no symbolic or abstract thought is present yet.

• Pre-operational stage: the child begins using language and symbols, but reasoning stays intuitive and egocentric, without systematic logical operations.

• Concrete-operational stage: the child can perform logical operations such as conservation and classification, but only on concrete, tangible material - not on abstract hypotheticals.

• Formal-operational stage: the child can reason abstractly, form and test hypotheses, and generalize principles beyond concrete particulars.

Since abstract thinking and generalization define the formal-operational stage specifically, that is the stage described in the question.