According to Jean Piaget, children in formal operational stage

Concept

Piaget's theory of cognitive development sets out four sequential stages: sensorimotor (birth to two years), preoperational (two to seven years), concrete operational (seven to eleven years), and formal operational (eleven years and above). The formal operational stage is marked by abstract, systematic thought — the ability to reason about hypothetical propositions, test possibilities methodically, and draw logical conclusions without needing concrete objects. Piaget termed this combination hypothetico-deductive reasoning and propositional thought.

Application to this question

The item asks what characterises children in the formal operational stage. That is precisely the capacity for hypothetico-deductive reasoning and propositional thought — systematically generating and testing hypotheses about abstract ideas rather than relying only on concrete, manipulable materials.

Cross-check: why the other traits belong to earlier stages

• Centration and irreversibility of thought are traits Piaget placed in reasoning from roughly two to seven years of age, where a child focuses on one salient feature of a situation and cannot mentally reverse a sequence of steps back to its starting state.

• The inability to conserve, classify, and seriate is also a limitation of that earlier reasoning; Piaget held that these three abilities are acquired between roughly seven and eleven years of age, not lost afterward.

• Make-believe and symbolic play — using one object or action to stand for another — emerges in reasoning from roughly two to four years of age, well before the abstract reasoning of later childhood develops.

None of these traits describes formal operational thought, which confirms that the capacity for hypothetico-deductive reasoning and propositional thought is the correct characterisation.