Final answer:
Petra's ability to hypothetically consider parents being held legally responsible for their children's crimes and argue both sides aligns with Piaget's formal operational stage, which begins around age 11.
Step-by-step explanation:
The ability to imagine what would happen if parents were held legally responsible for the crimes of their children and argue for or against it, regardless of personal opinion, is indicative of Piaget's theory of cognitive development, specifically the formal operational stage. This final stage, which typically begins around age 11 and continues into adulthood, is characterized by the ability to think abstractly and to process hypothetical situations. Unlike in the concrete operational stage, where thought processes are tied to concrete and tangible concepts, the formal operational stage allows for the exploration of abstract ideas and the formulation of arguments based on hypothetical circumstances.