Final answer:
The primary skill obtained by children during Piaget's formal operational stage is abstract thinking.
Step-by-step explanation:
During Piaget's formal operational stage, the primary skill obtained by the child is the ability to deal with abstract ideas and hypothetical situations. This stage generally starts from about age 11 and continues into adulthood. Children in this stage are not only able to think logically about concrete events, as they did in the concrete operational stage, but also use abstract thinking to problem-solve, consider alternative solutions, and test these solutions. Adolescents develop the capacity for abstract thought, enabling them to consider multiple perspectives, engage in debating ideas such as politics and ethics, and form new ideas, breaking away from the limitations of concrete thinking.